Pressure-driven transport of ternary electrolyte solutions with a common colon through charged membranes has been studied numerically for the homogeneous membrane model. Considerable changes in mixture composition have been predicted. The principal features of separation phenomenon are an increase in selectivity with decreasing feed ionic strength and mole fraction of more mobile counterions, though in the case of multiple-charged counterions with a lower mobility, opposite trends and dependencies with extrema also occur. It has been shown that adding electrolytes with less mobile counterions may improve the rejection of a starting electrolyte. The numerical analysis is in agreement with available analytical solutions and experimental data.