The invention relates to a dual-power input type three-gear tracked vehicle transmission. A power input mechanism, a dual-power input mechanism, a three-gear speed changing and braking mechanism, a differential type driving mechanism, a left side track driving mechanism, a right side track driving mechanism and a power output mechanism are installed on a box body. According to the transmission, a mechanical and hydraulic parallel-connection type dual-power input device, a differential type steering device, a three-speed gear shifting device, a built-in braking device are adopted, and gear rings of planetary gear mechanisms at the two sides of a driving wheel of the differential type steering device are controlled to rotate in the positive direction and in the negative direction through a hydraulic motor, so that driving efficiency and control convenience of a tracked vehicle can be improved effectively, the steering accuracy is improved, the rotation speed matching range of efficient driving is enlarged, the applications of the tracked vehicle are increased, the driving safety is improved, and the problems that in an existing tracked vehicle, the transmission efficiency is low, the driving cornering drag is high, the control is difficult, the mud accumulation is severe when the tracked vehicle turns on the edge of a field, and a braking device is complex are solved.