1. A rolling stand (100) for rolling a tube defining a rolling axis (X), comprising:
- a supporting structure (13),
- a roll-holder cartridge (3) configured to go from a working position inside the supporting structure (13), at the rolling axis (X), to an extracted position outside the supporting structure (13) with a transverse movement with respect to the rolling axis (X),
- three rolling rolls (2, 2', 2"), each of which having a respective support (6, 6', 6"), arranged in the roll-holder cartridge (3) with respective symmetry axes arranged at 120° with respect to one another, a first roll (2) of said three rolling rolls (2, 2', 2") having the respective symmetry axis arranged horizontally and the respective rotation axis arranged vertically,
- at least one hydraulic capsule (4, 4', 4") for each rolling roll, in order to adjust the radial position of the respective rolling roll (2, 2', 2"), fixed to the supporting structure (13) with a respective thrust cylinder, the at least one hydraulic capsule being configured to allow the respective thrust cylinder to cover a sufficient distancing stroke from the rolling axis (X) to allow to clear the path of the roll-holder cartridge (3) between the working position and the extracted position, characterized in that
the roll-holder cartridge (3) has two first walls (3', 3") facing each other, each first wall having an opening (10) equal to that of the other wall and defining three branches, wherein a first branch of the opening (10) has elongated shape along the horizontal symmetry axis of the first roll (2), and wherein a second branch and a third branch have a symmetry axis arranged at a 120° angle with respect to the symmetry axis of the first roll,
wherein the first branch is open at an end thereof coinciding with a side of the roll-holder cartridge (3),
wherein the supporting structure (13) has two second walls (13', 13") facing each other and there are three pairs of first axial resting surfaces (7, 7', 8, 8', 9, 9') on the inner surface of each of said two side walls (13', 13"), wherein each pair of first axial resting surfaces, when the cartridge is in said working position, is arranged so as to be in the first, second and third branch of the openings (10) of the first walls (3', 3") of the roll-holder cartridge and facing, along the direction of the rolling axis (X), to corresponding pairs of second axial resting surfaces (27, 27', 26, 26', 28, 28') integral with the support (6, 6', 6") of the rolling rolls, whereby the rolling forces directed along the rolling axis (X) are directly discharged from the supports (6, 6', 6") to the supporting structure (13).

2. A rolling stand (100) according to claim 1, wherein the first roll (2) and the respective support (6) can slide within the cartridge (3) in radial direction with respect to the rolling axis (X) and at least two pairs of first axial resting surfaces (7) and of second axial resting surfaces (27), arranged in the first branch of the openings (10) aligned with the first roll (2), comprise a supporting protrusion (50) adapted to support said first roll (2) and the respective support (6) vertically, in case of extraction of the roll-holder cartridge (3) from the supporting structure (13) caused by a jamming of the tube.
3. A rolling stand (100) according to claim 2, wherein said supporting protrusion is arranged either on a first axial resting surface (7) or on a second axial resting surface (27) of said at least two pairs of first axial resting surfaces (7) and of second axial resting surfaces (27).
4. A rolling stand (100) according to anyone of the preceding claims, wherein respective pairs of radial guides (20, 20', 21, 21', 22, 22') are provided at the first, second and third branches of the openings (10) of the first walls (3', 3"), and the roll-holder cartridge (3) is open at said pairs of radial guides (20, 20', 21, 21', 22, 22') to allow a sliding and a radial extraction of the supports (6, 6', 6") and of the respective rolling rolls (2, 2', 2").
5. A rolling stand (100) according to anyone of the preceding claims, wherein there is provided a recess (51), on an edge of the outer perimeter of the two first walls (3', 3") of the cartridge, adapted to house a corresponding protrusion (54) integrally fixed to the supporting structure (13), or vice versa.
6. A rolling stand (100) according to anyone of the preceding claims, wherein the first walls (3', 3") are arranged symmetrically with respect to each other, and each comprises:

- a first arm (40) provided with a first end portion (42) thinner than the body of said first arm (40) so that, considering the outer surface of the first walls (3', 3"), there is provided a recess in the passage from the body of the first arm (40) to said first end portion (42), at which a first shoulder (44) is provided;
- a second arm (41) provided with a second end portion (43) thinner than the body of said second arm (41) so that, considering the outer surface of the first walls (3', 3"), there is provided a recess in the passage from the body of the second arm (41) to said second end portion (43), at which a second shoulder (45) is provided;
- a body (46), which connects first arm (40) and second arm (41), provided with a V-shaped protrusion (47) thinner than said body (46) so that, considering the outer

surface of the first walls (3', 3"), there is provided a recess in the passage from said body (46) to said protrusion (47), at which a third shoulder (48) is provided.
7. A rolling stand (100) according to claim 6, wherein the protrusions defined by the first, second and third shoulders (44, 45, 48) have the same depth when measured in direction parallel to the rolling axis (X).
8. A rolling stand (100) according to claim 7, wherein the three pairs of first axial resting surfaces (7, 7', 8, 8', 9, 9') protrude from the inner surface of each of said two second walls (13', 13") of the supporting structure (13) and have a thickness along the rolling axis (X) essentially equal to the depth of the recesses defined by the first, second and third shoulders (44, 45, 48).
9. A rolling stand (100) according to anyone of the claims from 6 to 8, wherein when the roll-holder cartridge (3) is in the working position:

- two first pairs (8, 8') of the first axial resting surfaces, one first pair (8, 8') on each second wall (13', 13"), are at respective two first pairs of radial guides (21, 21') of the first walls (3', 3") and of respective two first pairs (26, 26') of the second axial resting surfaces;
- two second pairs (9, 9') of the first axial resting surfaces, one second pair (9, 9') on each second wall (13', 13"), are at respective two second pairs of radial guides (20, 20') of the first walls (3', 3") and of respective two second pairs (28, 28') of the second axial resting surfaces;
- and two third pairs (7, 7') of the first axial resting surfaces, one third pair (7, 7') on each second wall (13', 13"), are at respective two third pairs of radial guides (22, 22') of the first walls (3', 3") and of respective two third pairs (27, 27') of the second axial resting surfaces.

10. A rolling stand (100) according to anyone of the preceding claims, comprising a locking device (52, 53) for locking the position of the supports (6, 6', 6") within the roll-holder cartridge (3).
11. A rolling stand (100) according to anyone of the preceding claims, comprising a balancing device of the support (6) of the first roll (2), having vertical rotation axis, said balancing device being coaxial to the piston (24) of the corresponding hydraulic capsule (4), said piston being provided with a hammer head that is retractable and rotational with respect to the axis thereof.

12. A rolling stand according to anyone of the preceding claims, wherein the hydraulic capsules (4, 4', 4") are capsules controlled by means of a four-way servo-valve.
13. A method for releasing a rolling stand (100) according to claim 1, in case of jamming of a tube, such a method comprising, in order, the steps of:

a) blocking the two rolls (2', 2"), having symmetry axis inclined with respect to the horizontal, to the roll-holder cartridge (3) in a predetermined extraction position which does not hinder the path of the roll-holder cartridge (3) from the supporting structure (13) outwards,
b) releasing control extensions (5, 5', 5") from the three rolling rolls and positioning said control extensions so as to clear the path of the roll-holder cartridge (3),
c) pulling out the roll-holder cartridge (3) from the working position in the rolling stand to the extracted position by means of a side translation, leaving the yoke support (6) with the respective first roll (2), having vertical rotation axis, hanging from the supporting structure (13) of the rolling stand.