Quantum mechanical calculations of three-dimensional dissociative He+H-2 collisions are made using an infinite-order-sudden approximation. Vibrational and dissociative motions are treated accurately in the direct numerical method of Sakimoto and Onda [J. Chem. Phys. 100, 1171 (1994)]. The dissociation cross sections for all the initial vibrational states (v=0-14) are presented at the total energy E=8 eV. The dissociation cross section becomes much larger for higher v. The dissociation mechanism is different depending on the collision configuration. Two modes of dissociation are found to be important : The dissociation occurs (1) through the compressive action on the vibrational motion of H-2 and (2) through the expansive action on the vibrational motion.