Monte Carlo simulation and quasichemical theory are used to study the phase behavior of a lattice model for oil-water-amphiphile ternary systems. Several short amphiphiles with varying tail and head lengths are studied. Two- and three-phase coexistence regions are observed as well as the formation of microemulsions. In contrast to previous work on this model, quantitative phase diagrams are determined for both symmetric and asymmetric amphiphiles. The removal and regrowth of whole chains by configurational bias methods is used to help equilibration and sampling. Near quantitative agreement is found between quasichemical theory and our simulations except when one of the phases self-assembles or in the vicinity of a critical point. In these areas the quasichemical theory is still qualitatively correct.