In order to characterize the radar resolution, the original two dimensional ambiguity function is proposed to provide a conservation estimate for the joint distance-velocity resolution. In this paper, the ambiguity function is extended to three dimensions. So that, the array radar system can evaluate the radar resolution of the waveform in three dimensional of distance,velocity and azimuth angle. So the ambiguity function can represent the resolution ability between the interference and the target. The simulation experiment and performance analysis prove the validity and rationality of three dimensional ambiguity function.