Three-Parton Contributions to B --> M₁ M₂ Annihilation at Leading Order

We compute annihilation amplitudes for charmless B decays that are proportional to the three-parton twist-3 light meson distribution amplitude phi_{3M}(x1,x2) with an active gluon. Due to an enhancement from a quark propagator at the scale p^2 ~ mb\Lambda_QCD these terms occur at the same parametric order in alpha_s(mb) and 1/mb as the known leading order annihilation involving fB and twist-2 meson distributions. With our calculation the leading order annihilation amplitude is now complete. At lowest order in alpha_s the amplitudes are real and only O_{5-8} contribute. Using simple models we find that the three-parton and two-parton terms are of comparable size.