This work establishes the relation between three-body forces and the polarization P(r), for a group of interacting molecules A ... B ... C with weak or negligible charge overlap. Using nonlocal polarizability and hyperpolarizability densities, we derive new equations for the three-body forces and dipoles due to dispersion and induction-dispersion interactions. For clusters containing at least one polar molecule, we identify a nonadditive induction-dispersion energy that was not included explicitly in previous analyses of van der Waals cluster spectra. Numerical estimates show that this effect is significant for Ar-2 ... HCl and Ar-2 ... DCl. Separately, we determine the effects of linear and nonlinear induction, dispersion, and induction-dispersion interactions on the polarization P(r) : then we relate each contribution to P(r) to the corresponding component of the intermolecular force. Our work provides a classical electrostatic interpretation for all of the three-body forces at long range, including dispersion : The polarization of a molecular cluster produces these forces. This result holds even though particular interaction effects may generate a force but no net dipole, or a dipole but no net force.