The mid-infrared tunable lasers have a wide range of applications in military, medical, biological imaging, and scientific research fields. As the core working medium of mid-infrared tunable lasers, the gain medium plays an important role. In order to solve the bottleneck problem of Er³⁺ , Ho³⁺ , and Dy³⁺ ion activated mid-infrared laser crystals and obtain broadband fluorescence emission in the ~3 μm band, which is beneficial to realize a broadband tunable output, this work proposes a relevant optical performance optimization control scheme. Based on the existence of coupling interaction between rare earth ion levels, three coupling schemes of Er³⁺ /Ho³⁺ /Pr³⁺ ∶PbF₂, Yb³⁺ /Er³⁺ /Dy³⁺ ∶PbF₂ and Yb³⁺ /Ho³⁺ /Dy³⁺ ∶PbF₂ are proposed, and the optical properties are studied systematically. At the same time, an enhanced and widened 3 μm fluorescence emission is obtained, indicating that these kinds of crystals are expected to be gain materials for broadband tunable lasers.