A transmission unit includes: input shafts, each of the input shafts being provided with a shift driving gear thereon; output shafts, each of the output shafts being provided with a shift driven gear configured to mesh with a corresponding shift driving gear; a generator gear disposed on one of the output shafts; a motor power shaft; a first motor power shaft gear disposed on the motor power shaft and configured to rotate together with the generator gear; a second motor power shaft gear disposed on the motor power shaft and configured to rotate together with a shift driven gear; a reverse idler gear; and a reverse output gear configured to rotate together with a shift driving gear via the reverse idler gear. A power transmission system including the transmission unit and a vehicle including the power transmission system are also provided.