Low temperature photoluminescence spectra were measured under the selective excitation to the energy levels of Gd3+,Er3+ and Sm3+, respectively. Corresponding transitions were assigned with the help of fluorescence decay measurements. By assuming that the rare earth ion is in C-2v symmetry instead of the actual C-2, crystal field calculations were carried out based on the deduced experimental energy level data of Er3+ and Sm3+. The theoretical result is in good agreement with the experimental data.