In this paper, we investigate the unambiguous correct discrimination of three multi-copy quantum states with equal a priori probability. The measurement operators include the success operators and the correct operators. Using this scenario, we introduce the concept of the relative average correct probability, as a function of the average success probability. Based on the analysis of the relative average correct probability, the problem given in Chefles (Phys Rev A 64:062305, 2001) are solved completely. [ABSTRACT FROM AUTHOR]