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Using a two-site valence configuration interaction model, we reveal the diradical character (y) dependences of the third-harmonic generation (THG) spectra of singlet diradicaloids, which are described by the photon energy-dependent THG second hyperpolarizabilities γ. We particularly examine three- and two-photon resonance peaks associated with the dipole-allowed transition to the first excited state and with the dipole-forbidden transition to the second excited state, respectively. When increasing y, both peaks move to lower-energy region and become close to each other. Although the THG γ spectral intensities show resonance enhancements for any y value, we observe for THG γ the same bell-shape behavior as a function of y, as for the static γ, irrespective of the photon energy. [ABSTRACT FROM AUTHOR]