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.