Title of Invention: Spot Welding Method Technical Field [OOOl] The present invention relates to a spot welding method for joining a plurality of superposed steel sheets. Background Art [0002] In the past, the main welding method in the automobile field has been spot welding. Steel sheets have been joined by spot welding to assemble the chassis and produce parts. [0003] For example, in an automobile chassis, mild steel sheet excellent in shapeability and paintability has been used for outside panel parts with high design property. High strength steel sheet has been used for frame parts for ensuring inside strength and rigidity. Usually, mild steel sheet with a relatively thin sheet thickness and high strength steel sheet with a relatively thick sheet thickness are applied. [00041 Therefore, portions arise where thin mild steel sheet and thick high strength steel sheet are joined by spot welding. At this time, sometimes the nugget (weld metal) is not sufficiently formed at the interface between the thin sheet and thick sheet. [0005] PLT 1 proposes joining a plurality of steel sheets by resistance spot welding during which performing the resistance spot welding in two stages and performing the second stage of welding by a higher pressing force, lower current or same current, and longer conduction time or same conduction compared with the first stage of welding. [00061 However, with the method of PLT 1, the presslng force at the time of the first stage of welding is low, so there is a sheet gap between the steel sheets (gap between steel sheets). Further, if the sheet thickness ratio (=total sheet thickness (mm)/thinnest sheet thickness (rnrn)) is large, sometimes a nugget is not sufficiently formed at the interface of a thin sheet and thick sheet a.nd the desired joint strength cannot be 5 obtained. [00071 PLT 2 proposes a resistance welding method of high strength steel sheet for resistance welding a plurality of superposed steel sheets including at least one high strength steel sheet. Specifically, it proposes 10 a resistance welding method of high strength steel sheet comprising a first step of running current to form a nugget having a nugget size of 3dt to 5dt (t: smallest sheet thickness of steel sheets (mm)), a second step of then lowering the welding current, and a third step of 15 then running a welding current larger than the welding current of the first step to enlarge the nugget. [OOOS] In the method of PLT 2, the pressing force is increased in the second step, but if there is a sheet gap and the sheet thickness ratio is high, it is difficult to 2 0 secure the desired nugget size. [0009] PLT 3 proposes a resistance welding method of steel sheet for resistance welding a plurality of superposed steel sheets including at least one high strength steel sheet. Specifically, it proposes a 2 5 resistance welding method of high strength steel sheet comprising a first step of running current to form a nugget having a predetermined nugget size, a second step of then lowering the welding current, and a third step of then further running a welding current larger than the 30 welding current of the first step and imparting a pressing force larger than the pressing force of the first step to enlarge the nugget. [OOlOl In the method of PLT 3, to enlarge the nugget, at the third step, a pressing force larger than the 35 pressing force of the first step is imparted, but in the same way as the method of PLT 2, if there is a sheet gap and the sheet thickness ratio is high, it is difficult to secure the desired nugget size. [ OOll] PLT 4 proposes a method of spot welding three or more steel sheets having a gap G (<2(mm)) between steel sheets by multistage conduction comprising performing first conduction by upslope conduction then performing second conduction by the required conduction time and current value. [OOl2] In the method of PLT 4, the current of the second stage is made smaller than the maximum current of the first stage, so the interface between a thin sheet and thick sheet is insufficiently melted, a predetermined shape of nugget is not obtained, and the desired joint strength sometimes cannot be obtained. [0013] In this way, when superposing a plurality of steel sheets with a sheet gap and a high sheet thickness ratio, it is difficult to sufficiently melt the contact interface of a thin steel sheet and a thick steel sheet and sufficiently form the nugget and sometimes the desired joint strength cannot be obtained. Citation List Patent Literature [0014] PLT 1. Japanese Patent Publication No. 2005- 262259A PLT 2. Japanese Patent Publication No. 2010-207909A PLT 3. Japanese Patent Publication No. 2010-247215A PLT 4. Japanese Patent Publication No. 2013-188752A Summary of Invention Technical Problem [0015] In automobile members, in many cases steel sheets not only different in sheet thickness, but also different in strength such as mild steel excellent in paintability and shapeability and high strength steel are superposed and spot welded (resistance welded). (The plurality of superposed steel sheets is called a "set of sheets".) That is, thin sheets and thick sheets and mild steel and high strength steel are complicatedly assembled and welded. In automobile members, steel sheets pressed into various shapes are used. However, elastic deformation called "springback" occurs after the pressforming. In particular, in high strength steel, the springback is large and the sheet gap becomes large when 5 superposing steel sheets. When superposing and assemblinq steel sheets, sometimes a sheet gap of as much as 2 mm or so is formed. In spot welding, if the sheet gap is large and the pressing force is not sufficient, the contact area of the steel sheets before conduction becomes 10 smaller. In this case, the conduction path becomes narrow, that is, the current density becomes high, so there is the problem that melting rapidly proceeds locally and expulsion occurs. [00161 On the other hand, if spot welding a plurality 15 of superposed steel sheets, usually a nugget is formed at the approximate center in the sheet thickness direction. The main reason is that the more heat is removed by the spot welding electrode and the closer to the electrode, the more the temperature falls. Therefore, if placing a 2 0 steel sheet with a thin sheet thickness at the electrode side, it becomes difficult to form a nugget at the interface between the thin sheet and adjoining steel sheet. [0017] In the case of an automobile member, there are 25 portions where the thin mild steel sheet is arranged at the surface. The spot weldability with the high strength steel arranged adjoining it is becoming a problem. That is, the problem is the greatest when the sheet thickness ratio (sum of sheet thicknesses of superposed steel 30 sheets/sheet thickness of thinnest steel sheet) is large and the steel sheet with the thinnest sheet thickness is arranged at the electrode side in the spot welding. PLTs 2 and 3 propose a conduction pattern of three steps when spot welding high strength steel. That is, preliminary 35 conduction (first step) is used to secure the contact area of the steel sheets and eliminate expulsion at the initial stage of conduction. Conduction lowering the welding current (second step) and further conduction raising the welding current from the first step (third step) suppress inside expulsion occurring from the intermediate period to late period of the conduction. However, the sheet thickness is not recognized as a problem. The problem in the case of a large sheet thickness ratio cannot be solved. [OOlE] PLT 4 proposes a spot welding method when there is a sheet gap and arranging a steel sheet with a thin sheet thickness at the electrode side, but the welding current at the second conduction is lowered, so it is not possible to secure a suitable nugget size. Also, the welding time becomes longer, so this is not practical. [0019] The present invention has as its problem to solve these problems. That is, it has as its problem to obtain a suitable welded joint in spot welding a plurality of superposed steel sheets even when the gap between steel sheets (sheet gap) is as much as 2 mm or so and further the sheet thickness ratio (sum of sheet thicknesses of superposed steel sheets/sheet thickness of thinnest steel sheet) is large. In particular, the invention has as its object application to an automobile member and as its object provision of spot welding enabling application even when arranging a relatively thin steel sheet at the outermost layer (electrode-most side). Solution to Problem [0020] The inventors studied in depth the solution to the prob1em.A~ a result, they obtained the following discoveries. [0021] (a) They considered the mechanism of formation of a nugget in spot welding and discovered a conduction pattern enabling the suitable formation of a nugget without expulsion. Preliminary conduction: Conduction method promoting contact between steel sheets and reducing sheet gap by gradually increasing the welding current (for example, upslope conduction). If rapidly applying current, current would be locally carried and that part would rapidly melt resulting in expulsion, so this is a conduction method gradually running current (for example upslope 5 conduction) to avoid local heating. First conduction: Conduction method running a high and constant welding current while using the heat generated by the electrical contact resistance between steel sheets to melt a thin sheet and thick sheet so as 10 to suppress expulsion under conditions where preliminary conduction secures a certain contact area between the steel sheets and contact resistance remains. Second conduction: Conduction method making the current lower than the first conduction to suppress 15 inside expulsion while making the nugget grow mainly in the diametrical direction. Third conduction: Conduction method making the current higher than the second conduction and making the nugget grow not only in the diametrical direction, but 2 0 also especially in the sheet thickness direction when the nugget size becomes a certain extent of size at the second conduction step. [0022] (b) The inventors conducted numerous experiments to try to define conduction times and current 2 5 values in the different conductions using the sheet thicknesses and sheet gaps of the steel sheets as parameters. As a result, they were able to define suitable conduction times and welding currents in spot welding of steel sheets. 30 Note that, the "conduction time" is defined by the number of cycles corresponding to the frequency of the alternating current power supply. The amount of input heat is proportional to the integral of the input current values. These depend on the frequency of the alternating 35 current power supply. For example, if a 50Hz alternating current power source, 1 cycle = 1/50 seconds. [0023] Preliminary conduction tu>2xTa%Tg where, Ta: average sheet thickness (mm) (average sheet thickness Ta is defined by the value found by divi'ding the sum of the sheet thicknesses by 2) 5 Tg: maximum value of sheet gap (mm) tu: preliminary conduction time (cycles). The units of sheet thickness and sheet gap are mm (same below). The current value is at the most the same as the 10 current at the first conduction. [0024] First conduction lltl17xTa where, tl: constant conduction time of first conduction (cycles) 15 11110d(Ta) +2 where, I1 is current value at first conduction, units of kA . [0025] Second conduction lSt2 2 0 where, t2: second conduction time (cycles) 121104 (Ta) where, I2 is current value at second conduction, units of kA . [00261 Third conduction 25 5xTait3115xTa where, t3: third conduction time (cycles) 10I/(Ta)< 13 where, I3 is current value at third conduction, units of kA . 30 [0027] (c) When promoting nugget growth in the third conduction, rapid promotion of growth would be accompanied with the occurrence of expulsion. Therefore, in the third conduction as well, the inventors discovered that by setting a gradually increasing conduction pattern 35 (for example, upslope conduction), it is possible to suppress the occurrence of expulsion while promoting the growth of the nugget. KO0281 (d) The inventors discovered that when growing the nugget, rather -than by controlling just the current, it is possible to form a more suitable nugget by changing 5 the pressing force by the electrodes along with the nugget growth. In particular, the inventors discovered that by lowering the pressing force at the time of the third conduction, it is possible to weaken the cooling by the electrodes 10 and promote nugget growth in the sheet thickness direction (electrode direction). However, lowering the pressing force too much leads to the occurrence of expulsion, so it is preferable to make the pressing force 1/2 or more of the pressing force in the first conduction 15 or second conduction. [0029] (e) Furthermore, the inventors discovered that if providing a cooling step (time) between the first conduction step and second conduction step and/or between the second conduction step and third conduction step, the 2 0 weld zone as a whole is cooled and nugget growth can be suppressed once so as to be effective in suppression of expulsion. [0030] The present invention was made based on the above discoveries and has as its gist the following: 2 5 (1) A spot welding method joining a plurality of superposed steel sheets, the spot welding method comprising a preliminary conduction step of gradually applying current, 30 a first conduction step of running a constant current at a current value 11, a second conduction step next running current at a current value 12, and further a third conduction step of running current at a current 3 5 value 13, wherein I1>I2 and I2<13, where the units of 11, 12, and I3 are kA. (2) The spot welding method according to (1) wherein a sheet thickness ratio comprised of a ratio of a total of the sheet thicknesses of the plurality of steel sheets and a sheet thickness of the steel sheet with the 5 thinnest sheet thickness among the plurality of steel sheets is 4.5 or more. (3) The spot welding method according to (1) or (2) wherein the plurality of steel sheets are superposed so that the steel sheet with the thinnest sheet thickness 10 becomes the outermost side. (4) The spot welding method according to any one of (1) to (3) wherein the preliminary conduction step makes a welding current increase by upslope conduction. (5) The spot welding method according to any one of (1) 15 to (3) wherein the preliminary conduction step is pulsation conduction. (6) The spot welding method according to (4) wherein a conduction time of the upslope conduction of the preliminary conduction step is 1 to 30 cycles. 2 0 (7) The spot welding method according to (5) wherein a conduction time of the pulsation conduction of the preliminary conduction step is 1 to 10 cycles and an idling time is 1 to 5 cycles. (8) The spot welding method according to any one of (1) 2 5 to (7) wherein when designating a conduction time of the preliminary conduction step as tu, designating a conduction time of the first conduction step as tl, designating an average sheet thickness of a value found by dividing the sum of the sheet thicknesses of the steel 3 0 sheets by 2 as Ta, and designating a maximum value of the sheet gap comprised of the gap between steel sheets as Tg, the following relationships are satisfied: t u 2 2 ~ a ~ ~ g , l12) is run during the time t3. Melting of the steel sheets is further promoted by conduction by the current I3 and time t3, that is, enlargement of the nugget (enlargement in both of lateral direction and sheet thickness direction) is promoted. Contact and melting of the steel sheets proceed until the end of the second conduction step, that is, the conduction area sufficiently increases (current density falls), so the level of current I3 where expulsion occurs rises. Therefore, I3 can be made larger than 12. roo521 Furthermore, the inventors studied 13. That is, the current I3 should be set in the range where the nugget shape can be enlarged to the desired shape without causing expulsion. The inventors discovered that the current 13 should be set considering the average sheet thickness si.nce there is an effect. of the tol:al sheet thickness. As a result, it was learned that preferably 13>10d(~a). The upper limit of I3 does not have to be particularly limited. It is sufficient to set it so that no expulsjon occurs in spot welding. [0053] The sum of the conduction time t2 and the conduction time t3 (t2+t3) is an important indicator in terms of the contact between the steel sheets and growth of the nugget. Usually, in spot welding two sheets, it is known that at about lOxTa (cycles) or 10xTa+2 (cycles), the temperature rise tends to become saturated. The inventors sought the relationship between the average sheet thickness and the conduction time from this viewpoint. As a result, they discovered that it is sufficient to make the total conduction time of t2 and t3 5Ta to 15Ta. If the total conduction time of t2 and t3 is shorter than 5Ta, since the nugget does not sufficiently grow, suitable spot welding cannot be obtained. On the other hand, if the total conduction time between t2 and t3 is longer than 15Ta, the temperature distribution becomes substantially steady, the nugget size becomes saturated, and the productivity ends up falling. [0054] On the other hand, the second conduction step may be made longer, but the speed of growth of the nugget slows by the relatively small amount of the welding current. For this reason, there is a possibility that the welding time for obtain suitable spot welding will become longer. For this reason, t3 may be made longer than t2. [0055] At the third conduction step, the object is to make the nugget grow. It is particularly important to make it grow in the sheet thickness direction. Therefore, by not rapid heating, but by making the amount of heat input gradually increase in the same way as the preliminary heating, it is possible to suppress the occurrence of expulsion. The method of gradually increasing the input heat amount is not particularly limited, but it is preferable to make the welding current 5 increase by upslope conduction. For example, the third conduction current I3 may be made a function of the conduction time. For example, by making it a primary function of the conduction time, a monotonously increasing upslope conduction pattern is obtained (FIG. 6 10 and FIG. 7) . [0056] By making the third conduction step upslope conduction, it is possible to gradually grow a nugget, so this can also serve as the second conduction step. That is, in this case, it is possible to make the second 15 conduction step a short time. For example, it is possible to make t2=1 (cycle). 100571 Furthermore, the inventors discovered that when making the nugget grow in the sheet thickness direction, it is sufficient to reduce the cooling effect by the 2 0 electrodes. That is, the electrode itself is watercooled, so the temperature in the sheet thickness direction becomes lowest at the parts in contact with the electrode. For this reason, the nugget is difficult to grow in the sheet thickness direction. 2 5 [0058] Therefore, the inventors discovered that when making the nugget grow in the sheet thickness direction in the third conduction step, by decreasing the pressing force by the electrodes and decreasing the contact area between the electrodes and steel sheets to raise the 30 current density and simultaneously weaken the cooling effect by the electrodes, the nugget grows more effectively in the sheet thickness direction as well (FIG. 5 and FIG. 7). The lower limit value of the pressing force along with the decrease in the pressing 3 5 force is not particularly limited. However, if making the pressing force decrease too much, expulsion occurs, so the pressing force is preferably made half (1/2) of before the decrease or more. That is, if designating the pressing force before decreasing the pressing force, that is, at the first conduction step and the second conduction step, as P1 and the pressing force after 5 decreas~nq it P2, the presslng force should be 1/2PlSP212 and I2<13, where the units of 11, 12, and I3 are kA. Claim 2. The spot welding method according to claim 1 wherein a sheet thickness ratio comprised of a ratio of a total of the sheet thicknesses of said plurality of steel 2 0 sheets and a sheet thickness of the steel sheet with the thinnest sheet thickness among said plurality of steel sheets is 4.5 or more. Claim 3. The spot welding method according to claim 1 or 25 2 wherein said plurality of steel sheets are superposed so that the steel sheet with the thinnest sheet thickness becomes the outermost side. Claim 4. The spot welding method according to any one of 3 0 claims 1 to 3 wherein said preliminary conduction step makes a welding current increase by upslope conduction. Claim 5. The spot welding method according to any one of claims 1 to 3 wherein said preliminary conduction step is 35 pulsation conduction. Claim 6. The spot welding method according to claim 4 wherein a conduction time of the upslope conduction of said preliminary conduction step is 1 to 30 cycles. Claim 7. The. spot welding method according to claim 5 5 wherein a conduction time of the pulsation conduction of said preliminary conduction step is 1 to 10 cycles and an idling time is 1 to 5 cycles. Claim 8. The spot welding method according to any one of 10 claims 1 to 7 wherein when designating a conduction time of the preliminary conduction step as tu, designating a conduction time of said first conduction step as ti, designating an average sheet thickness of a value found by dividing the sum of the sheet thicknesses of the steel 15 sheets by 2 as Ta, and designating a maximum value of the sheet gap comprised of the gap between steel sheets as Tg, the following relationships are satisfied: tu>2~a'T~, iStil7Ta, and 11<10d(Ta) +2 where, the units of tu and tl are the numbers of cycles of alternating current applied and the units of Ta and Tg are mm. Claim 9. The spot welding method according to claim 8 wherein when designating a conduction time of said second conduction step as t2, the following relationships are satisfied: lit2 and 30 125104 ( ~ a ) where, the units of t2 are the numbers of cycles of alternating current applied. Claim 10. The spot welding method according to claim 8 or 3 5 9 wherein when designating a conduction time of said third conduction step as t3, the following relationships are satisfied: 5Ta