Master Integrals for Massless Three-Loop Form Factors: One-Loop and   Two-Loop Insertions
  The three-loop form factors in massless QCD can be expressed as a linear combination of master integrals. Besides a number of master integrals which factorise into products of one-loop and two-loop integrals, one finds 16 genuine three-loop integrals. Of these, six have the form of a bubble insertion inside a one-loop or two-loop vertex integral. We compute all master integrals with these insertion topologies. 
T. Gehrmann, G. Heinrich, T. Huber, C. Studerus