In order to show characteristic features in core-level spectroscopy, reflecting the localized- or itinerant nature of 4f electrons in Ce intermetallics, 3d x-ray photoemission spectroscopy, 3d x-ray absorption spectroscopy and 3d-4f resonant inelastic x-ray scattering (RIXS) are theoretically investigated for CeIn3 and CeSn3 with the AuCu3-type crystal structure. In this study, considering energy-band effects based on first-principles calculation within local density approximation, we employ an impurity Anderson model with full multiplet coupling. Moreover, in order to make a quantitative discussion, the basis configurations describing electron–hole pair creation are also included in the calculation within the configuration interaction scheme, which have been often neglected as a first approximation in previous studies. By comparing the calculated results for CeIn3 and CeSn3, we find two advantages of 3d-4f RIXS for a quantitative investigation of Ce intermetallics: (i) we can observe directly a binding energy in Kondo singlet formation by choosing proper polarization geometry of incident photons, and (ii) we can investigate an itinerant component related to Kondo singlet formation of Ce intermetallics by adjusting energy and polarization of incident photons.