The kinetic theory of diffusion over vacancies is used to obtain expressions for the fluxes of atoms in a three-component alloy. This hole gas approach allows us to express kinetic coefficients in terms of the coefficients of diffusion of labeled atoms from microscopic considerations. The conditions for the formation of nonequilibrium impurity shells are revealed by studying the kinetics of decomposition using these expressions. These shells can considerably inhibit the growth of precipitates and change the properties of the alloy.