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.
