A gear shifting device, a transmission, and an all-terrain vehicle are disclosed. The gear shifting device includes: a drive motor having an output shaft fixed with a driving toothed wheel; a transmission drum having a first end fixed with a driven toothed wheel and a second end provided with a gear contactor; a gear sensor having a working surface in contact with the gear contactor; and an electronic control unit electrically coupled to the drive motor. The electronic control unit outputs a drive signal to the drive motor based on a gear shifting instruction, the drive motor rotates based on the drive signal, and the output shaft drives the driving toothed wheel to rotate; the transmission drum and the driven toothed wheel rotate along with rotation of the driving toothed wheel; and the gear contactor rotates to contact one of four contacts corresponding to the gear shifting instruction.