Massive three-loop form factor in the planar limit
  We compute the three-loop QCD corrections to the massive quark-anti-quark-photon form factors $F_1$ and $F_2$ in the large-$N_c$ limit. The analytic results are expressed in terms of Goncharov polylogarithms. This allows for a straightforward numerical evaluation. We also derive series expansions, including power suppressed terms, for three kinematic regions corresponding to small and large invariant masses of the photon momentum, and small velocities of the heavy quarks. 
Johannes Henn, Alexander V. Smirnov, Vladimir A. Smirnov, Matthias   Steinhauser
hep-ph