A transmission unit for a vehicle is provided. The transmission unit includes: a plurality of input shafts, each of the input shafts being provided with a shift driving gear thereon; a plurality of output shafts, each of the output shafts being provided with a shift driven gear configured to mesh with a corresponding shift driving gear; a motor power shaft; first and second motor gears fitted over the motor power shaft; a motor synchronizer disposed on the motor power shaft and between the first and second motor gears; in which the first motor gear is configured to rotate together with one of the input shafts; the second motor gear is configured to rotate together with one of the output shafts. A power transmission system including the transmission unit and a vehicle including the power transmission system are also provided.