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The importance of fluoro-perovskites, especially KMgF3 and NaMgF3, for the industrial technology is indisputable. The flagrant lack of adequate information on K[Formula: see text]Na[Formula: see text]MgF3 motivated us to undertake a comprehensive study of their behavior under the effect of hydrostatic pressure. The scope of the study includes theoretical prediction in order to analyze and provide more information on the structural, mechanical, electronic and optical properties under the effect of pressure and their mechanical stability using all-electron self-consistent full-potential linearized augmented plane waves plus local orbital (FP-(L)APW[Formula: see text]lo) method. Structural and elastic properties such as the lattice parameter, bulk modulus, its pressure derivative and the cohesive energy of KMgF3, K[Formula: see text]Na[Formula: see text]MgF3 and NaMgF3 as well as the effect of pressure on the values of lattice parameter, elastic constants and their related parameters have been predicted using GGA-PBE and GGA-PBEsol. These functionals have shown great success in the prediction of such properties and the related found results are closer to those available. The modification of the Becke–Johnson potential developed by Tran and Blaha has remedied the problem of underestimation of bandgap calculation by the GGA. This gave us an opportunity to predict bandgaps of these materials using this potential. For KMgF3, the found result (using this potential) is very close to that of experimental and we firmly believe that our values for K[Formula: see text]Na[Formula: see text]MgF3 and NaMgF3 (using this same potential) will also be closer once their bandgaps will be calculated. Dielectric functions of KMgF3, K[Formula: see text]Na[Formula: see text]MgF3 and NaMgF3 under hydrostatic pressure have been studied with several functionals (GGA-PBE, GGA-PBEsol and GGA-EV).

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