  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.
