We report an extensive analytical and numerical study of the trimolecular system A + 2X reversible arrow 3X on lattices with restricted geometry, including fractal sets. It is shown that the asymptotic value of [A]/[X] attained starting from a lattice completely covered by X is largely determined by the mean coordination number, while fractal dimension seems to play little role. If a small external flux of A is allowed, the behavior is no longer dominated by coordination number, but by the existence of stable configurations of A-sites.