The non-diagonal correlators of vector and
scalar currents are considered at three-loop order in QCD. The full
mass dependence is computed in the case where one of the quarks is
massless and the other one carries the mass M. As applications we
consider the single-top-quark production via the process $q\bar{q}\to
t\bar{b}$ and the decay rate of a charged Higgs into hadrons.
In both cases the computed NLO corrections are shown to be numerically
much less important than the leading ones.