Recent field-theoretic simulations of symmetric ternary blends of A- and B-type homopolymers with AB diblock copolymer [Vorselaars, B.; et al. Phys. Rev. Lett., 2020, 125, 117801]predicted three-phase coexistence between bicontinuous microemulsion (BEE) and two homopolymer-rich phases. The present study begins by repeating their grand-canonical simulations over longer durations in simulation boxes of different sizes to ensure that the prediction was not an artifact of nonequilibrium effects or finite system sizes. The coexistence is then demonstrated in canonical simulations, where the three phases are explicitly separated by interfaces. From those simulations, we extract the interfacial widths, the interfacial tensions, the domain size in the B mu E phase, and the copolymer concentration in each phase. The latter results are used to improve the accuracy of the previous phase diagram.