Microscopic rate constants for Tb(5D3) cross relaxation have been determined as a function of temperature in symmetrical Tb3+−Tb3+ pairs in CsCdBr3, CsMgBr3 and CsMgCl3. The temperature dependence and maximum efficiency of cross relaxation is very different in the three hosts, despite the fact that the energy-level structures and luminescence spectra are quite similar. The difference in cross relaxation behavior in the three systems is due to the fact that the degree of resonance is the dominant factor in determining transfer efficiencies, easily outweighing the influences of general transition strengths and donor−acceptor separations. Only donor and acceptor transitions with significant overlap of the zero-phonon spectral lines participate in active transfer mechanisms. The temperature dependence of cross relaxation is due to the changing thermal populations of the initial crystal-field levels of resonant donor and acceptor transitions. All transitions in resonance involve a “hot” crystal-field transition, either on the donor, on the acceptor, or on both. The marked differences in the temperature dependence of cross relaxation in the three systems are attributed to slight differences in crystal-field energy-level structures, which, in turn, result in different crystal-field transitions being in resonance in the three systems. Although phonon-assisted processes are not discounted, they do not appear to have a significant impact on the temperature dependence of cross relaxation. Crystal-field analyses for Tb3+ in symmetric pairs in the three hosts are also reported.