  Three-loop corrections to the scalar and pseudo-scalar current correlator are
calculated. By applying the large momentum expansion mass terms up to order
(m^2/q^2)^4 are evaluated analytically. As an application O(\alpha_s^2)
corrections to the decay of a scalar and pseudo-scalar Higgs boson into top
quarks are considered. It is shown that for a Higgs mass not far above the
$t\bar{t}$ threshold these higher order mass corrections are necessary to get
reliable results.
