  We compute three-loop corrections to the singlet form factors for massive
quarks using a semi-analytic method which provides precise results over the
whole kinematic range. Particular emphasis is put on the anomaly contribution
originating from an external axial-vector current. We also discuss in detail
the contribution for a pseudoscalar current and verify the chiral Ward identity
to three-loop order. Explicit results are presented for the low- and
high-energy regions and the expansions around threshold.
