Nonlinear optical susceptibilities of conjugated polymers are studied in the Su-Schrieffer-Heeger model supplemented by long-range Coulomb interactions, taking account of the formation of excitons and the influence of disorder. In the calculated third-harmonic generation spectrum, on the high energy side of the main peak (due to the lowest singlet exciton) a second peak (due to the three-photon resonance to the electron-hole continuum) is found that is strong enough to remain significant even in the presence of disorder. The results are in good agreement with experiments in polydiacetylene and polythiophene.