The radiative and nonradiative transitions of the first three excited levels of Er3+ doped in Ga2S3–GeS2–Sb2S3 glass were investigated. Particularly, we focused on the 4I9/2 and 4I11/2 which are the initial and final levels of mid-infrared emission around 4 µm. The effects of energy transfer from these levels to co-doped rare-earth ions, Yb3+ and Nd3+, were estimated. The population of the final level, 4I11/2, was efficiently quenched by the energy transfer to the Yb3+, 2F5/2 level. This leads to the population inversion between the initial and final levels. In the Nd3+-co-doped glass, on the contrary, the initial level, 4I9/2, was more rapidly quenched than the final level, 4I11/2, through the energy transfer to Nd3+, 4F5/2 + 2H9/2.