Fig. 6. The three-drug parameter surface and three-drug axes seen at the base. Representative surfaces for U⁵⁰as a function of θ^Band θ^C. The three corners of the triangular surface represent the value of U⁵⁰(θ^B, θ^C) for drug A alone, drug B alone and drug C alone. The three edges represent the value of U⁵⁰(θ^B, θ^C) for the paired combinations for A and B (labeled θ^AB), A and C (labeled θ^AC), and B and C (labeled θ^BC). The surface represents the value of U⁵⁰(θ^B, θ^C) for the three-drug combination. (Top ) Shows additive interaction for all three drugs. (Middle ) Shows synergistic interaction between drug A and drug B and between drug A and drug C, but shows the additive interaction between drug B and drug C. (Bottom ) Shows synergistic interactions among all three drugs.