A study was conducted of optical emission spectra, taken at a region near the source of the evaporation of Ag in a low pressure helium gas (100 Torr). A new Ag-3 emission band system was identified. Spectroscopic constants for an excited and a lower state (the ground state) in the gas phase were derived as follows : nu(00)=19809.4+/-2.0 cm(-1), omega(1)’=106.8+/-1.0 cm(-1), x(11)’=-0.2+/-0.5 cm(-1), omega(1) "=129.0+/-1.0 cm(-1), and x(11)"=-0.1+/-0.5 cm(-1) for the (Ag2Ag)-Ag-107-Ag-109 trimers. Spectroscopic cm constants for three other isotope species were also derived. The lower state was the B-2(2) ground state.