Many experimental studies, over the past two decades, have constantly reported a critical behavior for the transition from the smectic- A phase of liquid crystals to the hexatic- B phase with non- XY critical exponents. However, according to symmetry arguments this transition must belong to the XY universality class. Using an optimized Monte Carlo simulation technique based on the multihistogram method, we have investigated the phase diagram of a coupled XY model, proposed by Bruinsma and Aeppli [Phys. Rev. Lett. 48, 1625 (1982)], in three dimensions. The simulation results demonstrate the existence of a tricritical point for this model, in which two different orderings are established simultaneously. This result verifies the accepted idea that the large specific heat anomaly exponent observed for the SmA-HexB transition could be due to the occurrence of this transition in the vicinity of a tricritical point.