Electric dipole assisted third harmonic generation (THG) by a CO2 laser over a rippled Ag and n-InSb metallic surface is investigated theoretically. The laser of frequency is incident over corrugated metal films using a Kretschmann attenuated total reflection technique that gives rise to electron oscillation. It exerts ponderomotive force at that couples with electron velocity at to produce a non-linear current at . Enhancement in the electric field of the third harmonic in n-InSb is more than Ag by an order of two due to the high non-linear current density. The n-InSb excitation area of the third harmonic is noticed to be significantly larger than the spot size of the incident beam. The proposed scheme of surface enhanced THG provides a new avenue to upgrade non-linear infrared spectroscopy and bio-molecule imaging.