A working fluid for heat cycle, a composition for a heat cycle system containing the working fluid, and a heat cycle system employing the composition are provided. The working fluid contains trifluoroethylene, 2,3,3,3-tetrafluoropropene, and difluoromethane. A proportion of the total amount of trifluoroethylene, 2,3,3,3-tetrafluoropropene, and difluoromethane based on the entire amount of the working fluid is higher than 90 mass % and at most 100 mass %. Based on the total amount of trifluoroethylene, 2,3,3,3-tetrafluoropropene, and difluoromethane, the proportion of trifluoroethylene is at least 10 mass % and less than 70 mass %, the proportion of 2,3,3,3-tetrafluoropropene is at most 50 mass %, and the proportion of difluoromethane is higher than 30 mass % and at most 75 mass %. The working fluid can replace R410A and has a low global warming potential and high durability.