The best chemical ordering structures of 79-atom trimetallic PdnAg(60-n)Pt19 nanoalloys with truncated octahedron (TO) geometry were optimized and local relaxations were performed by using Monte Carlo Basin-Hopping algorithm within Gupta potential. The mixing energy variations were calculated to compare the relative stability. The lowest excess energy value was obtained at the compositions of Pd25Ag35Pt19 at Gupta level. In addition, a detailed investigation of local atomic pressure has been carried out. The factors affecting local atomic pressures were discussed.