The invention relates to a three-speed modular transmission device. The transmission device adopts a modular design; one module is a transmission unit; an input shaft, two clutches, two brakes and a planet row are integrated in the module; and change of three-speed transmission ratios can be realized through control of different combinations of the clutches and the brakes. The planet row serves asan important transmission part, adopts a coplanar double-inner engagement planet gear transmission form, and mainly comprises a center gear, a coplanar gear ring, an outer gear ring and a planet carrier. Inner and outer gear teeth of the coplanar gear ring form inner engaged gear engagement forms with the center gear and the outer gear ring. The clutches and the brakes are control modules of themodule, and mainly comprise clutch oil cylinders, brake oil cylinders, piston friction discs, middle friction discs, clutch fixed friction discs, brake friction discs and the like. All the friction discs adopt multi-conical groove structural designs, so that the transfer torque volume is increased, and miniaturization and lightness of the transmission device are achieved.