Metal halide perovskites are recently a ttracting strong attention due to their potential in solar cells, LEDs, and lasers. Here, we demonstrate the broad spectral tuning of the optical gain characteristics of triple cation (containing methylammonium, formamidinium and cesium) mixed-halide perovskite thin films. We explicitly study the interrelation between amplified spontaneous emission (ASE) thresholds, the operational stability of the gain, and the material composition. The incorporation of cesium and a deficiency of lead in the precursor solutions is found to be crucial for low ASE thresholds and the improved stability
of mixed-halide perovskites. We tune the photoluminescence in mixed-halide perovskites between peak wavelengths of 510 and 790 nm by exchanging the halide from iodide to bromide and chloride in small steps of 10%, while preserving the narrowband emission below a linewidth of 130 meV for all mixtures. The optical gain under ns-excitation can be tuned over a significant portion of this spectral window; we ... mehrobserve ASE emission in regions between 545 to 555 nm and 680 to 810 nm. This is a significant step towards perovskite lasers operating through a broad portion of the visible to near infrared spectrum.