This paper describes the basic concept of our multimodal sensor control system for 3-Degree-of-Freedom transradial prosthesis. The target of developing the controller is to reduce the mental effort of planning operating multiple joints in the conventional multifunctional myoelectric controller and reduce the compensating motion of conventional myoelectric prosthesis. An accelerometer is installed in the socket and the angles of the gravitational force are computed to drive the pronation/spination joint and the palmar flexion/dorsifelxion joint of the prosthesis. A threshold On/Off control using the posture information is implemented with the triggering of a co-contraction EMG signal. Through experiment with able-body subjects, we confirmed that this controller has a potential of reducing compensating shoulder movements for pick-raise-place tasks, when compared to the task conducted with conventional locked-wrist prostheses. Yet modification is required for stability.