We report on recent progress in calculating the three loop QCD corrections of the heavy flavor
contributions in deep--inelastic scattering and the massive operator matrix elements of the variable flavor
number scheme. Notably we deal with the operator matrix elements $A_{gg,Q}^{(3)}$ and $A_{Qg}^{(3)}$ and
technical steps to their calculation. In particular, a new method to obtain the inverse Mellin transform
without computing the corresponding $N$--space expressions is discussed.