The structure of 3:3 and 1:3 electrolyte solutions at various concentrations and several cation/anion size ratios has been analyzed in terms of triplet and pair correlation functions, by means of simulation and a triplet integral equation theory derived from the inhomogeneous Ornstein-Zernike equation. The interaction model consists of a truncated and shifted Coulomb plus the Ramanathan-Friedman repulsive core. Concentration and size and charge asymmetry are found to induce changes in the triplet structure beyond those predicted by the simple superposition approximation, which are, however, correctly reproduced by the triplet integral equation.