It is still a challenge to design a hand prosthesis with a consideration of multi- motions and light weight. In this paper, by analyzing 16 commonly-used motions of human hands, a trade-off plan between weight saving and the number of motions of a hand prosthesis is studied. We determine the functions to be implemented as a constant interlock mechanism of four fingers. An adaptive mechanism is applied for the thumb, and the symmetric series elastic actuator is used for the arching of metacarpal. With such a design, a prosthetic hand was designed just using three motors embedded in the palm, which has weight of 132.1 g and could perform 11 motions. The grasping stability and operability of the hand prosthesis were confirmed with intuitive myoelectric control based on a neural network algorithm in the subject experiments.