  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.
