An actuator, a robot arm and a robot are disclosed. The actuator includes: a housing; a motor, including a motor stator and a motor rotor, wherein the motor stator is disposed on the housing, the motor rotor is rotatably connected to the housing, and the motor rotor covers the motor stator; a position encoder, disposed on the motor rotor; a motor driver, disposed on the housing, and electrically connected to the motor; a reducer, disposed on the housing, and parallelly disposed with the motor; and a transmission mechanism, connected to the motor rotor and the reducer respectively; wherein the reducer is configured to adjust a rotation speed output from the motor rotor.