A transmission unit includes: input shafts; output shafts configured to transmit with a corresponding input shaft via gears; a reverse output gear fitted over one output shaft; a reverse synchronizer; a reverse shaft configured to rotate together with a input shaft and a reverse output gear; a motor power shaft; a first and a second motor gears fitted over the motor power shaft; the second motor gear configured to rotate together with a shift driven gear; and a motor synchronizer. A power transmission system including the transmission unit and a vehicle including the power transmission system are also provided.