  We consider the three-loop singlet diagrams induced by axial-vector, scalar
and pseudo-scalar currents. Expansions for small and large external momentum
$q$ are presented. They are used in combination with conformal mapping and
Pad\'e approximations in order to arrive at results for the polarization
functions valid for all $q^2$. Results are presented for the imaginary parts
which are directly related to physical quantities like the production of top
quarks or the decay of scalar or pseudo-scalar Higgs bosons.
