We investigated a Ge-based ternary alloy thin film, Ge1-x-ySixSny, for synthesis, the control of carrier concentration, and thermoelectric properties, using an advanced solid-phase crystallization. The addition of small amounts of Si and Sn in Ge lowered the thermal conductivity while maintaining high power factors, resulting in higher dimensionless figure of merit than most of the environmentally friendly thin-film thermoelectrics.