To improve the performance of materials, we employ first-principles techniques based on density functional theory (DFT) implemented in the Wien 2k code to study perovskites Ba3CaAx2O9. We used the Birch-Murnaghan fit, tolerance factor, and formation energy to determine the structural stability of these materials. Structurally stable compounds Ba3CaAx2O9 have a tolerance factor of 0.7, 1.1, and 0.9, respectively, and a formation energy of −3.04 eV, −1.98, and −1.12 eV. We used the proposed mBJ+U potential to assess the properties of the present materials, providing insight into their nature. Band gaps of 1.5–3.2 eV for Ba3CaAx2O9 (Ax = Nb, Mo, Rh) indicate that they are indirect semiconductors. We also calculate the density of states (DOS) for both materials, and our results for the energies of the band gaps agree with those from the band structure. Absorption and optical conduction are observed to occur in the UV region, and all investigated materials are transparent to low-energy photons. Our investigation into the optical characteristics of these materials suggests they could be promising candidates for application in optoelectronic devices. Finally, the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit have been calculated to investigate the thermoelectric efficiency of the triple perovskites based on semi-classical Boltzmann theory. The findings suggest that triple perovskites may be a promising candidate for optoelectronic and thermoelectric applications.