We present a combined theoretical and experimental study of intramultiplet transitions in collisions of C(P-3) and Si(P-3) with He. Relaxation rate constants have been measured using the CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) technique at 15 K for C(P-3)+He and over the 15-49 K temperature range for Si(P-3)+He. Three sets of interaction potentials have been generated for the (3)Sigma(-) and (3)Pi electronic states of CHe and SiHe systems. Quantum-mechanical calculations have been performed on these potential curves for the spin-orbit relaxation and excitation. Cross sections and rate constants are very sensitive to the interaction potential. For the Si(P-3)+He system, an overall good agreement between the theoretical and experimental rate constants is found with the best quality interaction potential, while for the C(P-3)+He system experimental rate constants are much smaller than the theoretical ones.