  We compute three-loop corrections to the process $gg\to HH$ originating from
one-particle reducible diagrams. This requires the computation of two-loop
corrections to the gluon-gluon-Higgs vertex with an off-shell gluon. We
describe in detail our approach to obtain semi-analytic results for the vertex
form factors and present results for the two form factors contributing to Higgs
boson pair production.
