It is shown that the optical Eabi precession model developed by Feynman, Vernon and Hellwarth for two-level systems can be extended not only to discuss the two-photon resonant coherent interaction, but also to the case of three photons. Using this model and the adiabatic following approximation, the third-order polarization expression in an intense and (or) resonant optical field is obtained, which differs from that obtained by perturbation theory. The saturation phenomena in the third-order resonant non-linear effects are also discussed.