Tm2O3–Tb4O7–Sm2O3 tri-doped glass ceramics containing eulytite-type structure were prepared by melt-annealing and crystallization. Controlling crystallization can be achieved by adjusting the heat treatment temperature or time. XRD Rietveld shows that rare earth ions are doped into the Ba3Gd(PO4)3 lattice. A significant reduction in optical band gap values was observed in glass ceramics after heat treatment. Emission spectra and decay curves verified the energy transfer from Tm3+ and Tb3+ to Sm3+, and white light emission was achieved by adjusting the doping concentration of Sm2O3. Based on the temperature-dependent properties of the fluorescence intensity ratios of Sm3+/Tm3+ and Sm3+/Tb3+, the maximum value of the relative sensitivity (Sr) was calculated to be 2.36% K−1. The results show that Tm2O3–Tb4O7–Sm2O3 tri-doped glass ceramic can be a candidate material for w-LEDs and optical temperature measurement.