Photoluminescence excitation (PLE) and emission spectra (PL) of undoped (Sr, Ca)3(Y, Lu)2Ge3O12 as well as Eu3+- and Ce3+-doped samples have been investigated. The PL spectra show that Eu3+ enters into both dodecahedral (Ca, Sr) and octahedral (Y, Lu) sites. Ce3+ gives two broad excitation bands in the range of 200−450 nm. First-principle calculations for Ce3+ on both dodecahedral and octahedral sites provide sets of 5d excited level energies that are consistent with the experimental results. Then the vacuum referred binding energy diagrams for (Sr, Ca)3(Y, Lu)2Ge3O12 have been constructed with the lanthanide dopant energy levels by utilizing spectroscopic data. The Ce3+ 5d excited states are calculated by first-principles calculations. Thermoluminescence (TL) glow curves of (Ce3+, Sm3+)-codoped (Sr, Ca)3(Y, Lu)2Ge3O12 samples show a good agreement with the prediction of lanthanide trapping depths derived from the energy level diagram. Finally, the energy level diagram is used to explain the low thermal quenching temperature of Ce3+ and the absence of afterglow in (Sr, Ca)3(Y, Lu)2Ge3O12.