An analysis of the optical properties of compounds based on 3d elements provides valuable information about the electronic structure of the ground state and low-energy excitations. Thus, we show that the analysis of charge-transfer d-d excitons in the dielectric antiferromagnetic phase of cuprates and metastable low-energy electron-hole EH dimers being a result of their evolution after electron-lattice relaxation, turns out to be very fruitful not only for describing linear and nonlinear optical properties and photoinduced effects, but also to develop a promising model of charge triplets to describe the low-energy electronic structure and T-x phase diagrams of active CuO2 planes in cuprates of the T-La2CuO4 or T’-Nd2CuO4 type, as well as NiO2 planes in nickelates of the RNiO2 type, and their evolution with changes in the main energy parameters.