We show by molecular dynamics simulations on three systems (B/A=Pd/Ag, Cu/Ag, and Ni/Ag) that three-shell metallic nanoparticles made by a core of a metal A, an intermediate shell of metal B and an external shell of metal A (A-B-A nanoparticles) can be grown by deposition of B atoms onto an A core. The growth of the intermediate B shell is triggered by the fact that the most favorable positions for isolated B impurities inside A clusters are located just one layer below the cluster surface.