Phase modulation is widespread in communication systems. A lot of such modulation types are used: 2-PSK, 4-PhM, 8-PSK, 18-PSK and so on. In space communication systems, such as INTELSAT, DOSAT, DSCS, SKYNET signals with 8-PSK modulation are exploited. Using these signals three bites are transmitted in one message and in the receiver there are identified the phase state corresponded to transmitted bites. Therefore there produced an estimation of three bits at ones. In the article there synthesized the optimal algorithm of each bit estimation separately according to maximum a posteriori probability criterion. Formulas for error probability are derived. Several types of encoders at the transmitter are considered. The best encoder is revealed for which there are provided low less error probability of each bit estimation