Polymer-protected trimetallic nanoparticles having a triple core/shell structure, such as Pd-core/Ag-inter layer/Rh-shell and Au-core/Pt-inter layer/Rh-shell structures, have been synthesized by a self-organizing method (physical mixture at room temperature). These trimetallic nanoparticles have much higher catalytic activities than the corresponding monometallic and bimetallic nanoparticles as well as trimetallic nanoparticles having different structures.