  We present a determination of a new class of three-loop Feynman diagrams
describing heavy-to-light transitions. We apply it to find the O(alpha_s^2)
corrections to the top quark decay t -> bW and to the distribution of lepton
invariant mass in the semileptonic b quark decay b -> ul\nu. We also confirm
the previously determined total rate of that process as well as the O(alpha^2)
corrections to the muon lifetime.
