We study the photon-quark-quark and Higgs-gluon-gluon form factors for
on-shell massless quarks and gluons in perturbative QCD. Previous third-order
results for the quark case are extended by calculating the fermion-loop
contributions up to the finite terms in dimensional regularization. For the
gluon case the complete set of infrared poles at three loops is derived. Using
the exponentiation of the form factor, the latter results are employed to
extract a function entering the infrared factorization of general third-order
amplitudes. We evaluate the infrared finite absolute ratio of the time-like and
space-like gluon form factors up to the fourth order in the strong coupling
constant. The result supports previous indications that the perturbative
expansion of the Higgs boson production rate at the LHC is under control.Comment: 9 pages, LaTeX. FORM file of main results appended to sourc