Abstract: In this paper, we investigate a class of three-heat-source heat pumps, in which the only irreversible process is heat conduction. It is shown that the endoreversible three-heat-source heat pump is the most optimal one. Moreover,the relation between the optimal coefficient of performance and the rate of heating is induced. Thus the fundamental effect of thermal resistance on the optimal performance of a three-heat-source heat pump is expounded.The conclusions in this paper are more realistic than those in classical thermodynamics.