Although the second virial coefficient has been used frequently to test intermolecular potentials, the direct calculation of third virial coefficients has only been attempted for atomic or simple (with linear symmetry) multipolar fluids. In this paper a general approach is described for the numerical evaluation of the third virial coefficient for molecular systems. Specific results for the second and third virial coefficients for several different water models are reported and compared with experimental data; critical points are also estimated. A polarizable model is found to be superior to standard effective liquid-phase potentials in reproducing the equation of state for steam.