Record performance Metal-Halide Perovskite (MHP) based solar cells have been achieved by incremental optimization of deposition procedures based on spin-coating. We here provide unprecedented insight into the formation process of MHP thin films of the “triple cation” (Cs,MA,FA)Pb(Br,I)3 perovskite from multi-modal in-situ optical process monitoring during spin-coating and annealing. This report details small-footprint fiber-optics based optical spectroscopy setup that enables monitoring of thin-film formation processes by UV-Vis reflectance and photoluminescence spectroscopy with a sub-second time resolution. Complementary information can be obtained from optical features during different stages of film formation: 1) During the first, flow regime dominated, stage of spin-coating, the wet-film thinning can be analyzed from UV-Vis interference, 2) the onset of bulk perovskite formation is clearly observed from the evolution of the semiconductor absorption edge, and 3) Photoluminescence (PL) measurements provide complementary information on nucleation and growth processes. We here provide a comprehensive picture that rationalizes the conditions to obtain a high quality “triple cation” perovskite thin-film during spin-coating and subsequent annealing.