The polarization properties of stimulated three-photon scattering in a three-level atomic system are presented. In our investigation of the stimulated emission spectrum of Na, we find that interference between the amplitudes of the resonant contributions of the Na doublet states results in interesting polarization phenomena. Particularly striking is the behavior when the laser is tuned near the dispersion-free point. With the laser linearly polarized the stimulated three-photon scattering is polarized perpendicular to the laser polarization. For circularly polarized excitation, total extinction of three-photon scattering is observed.