We examine a bimetallic alloy, Pt3Co, modified by addition of Iridium. The ternary alloy, Pt2IrCo, exhibits a strong Pt-segregation tendency to form a Pt-skin surface. The adsorption properties and electrochemical stability of Pt2IrCo(111) surface are studied using density functional theory and compared with Pt(111) and Pt3Co(111) surfaces. It is found that the Pt-skin surface of Pt2IrCo(111) has similar electrochemical stability to the Pt3Co(111) Pt-skin surface. The binding strength of OH is much weaker on the trimetallic Pt-skin surface, suggesting a possible activity enhancement for the oxygen reduction reaction. Further analyses reveal the origin of the catalytic activity and stability enhancement for the alloy.