  Using a diffusion Monte Carlo (DMC) technique, we calculated the phase
diagram of 3He adsorbed on a first solid layer of a molecular hydrogen isotope
(H2,HD and D2) on top of graphite. The results are qualitatively similar in all
cases: a two-dimensional gas spanning from the infinite dilution limit to a
second-layer helium density of 0.048 +/- 0.004 Ang^{-2}. That gas is in
equilibrium with a 7/12 commensurate structure, more stable than any
incommensurate triangular solid of similar density. These findings are in
reasonably good agreement with available experimental data.
