A three-way diverter assembly with a contoured valve is provided. The three-way diverter assembly includes a housing having a first inlet, a second inlet, a first outlet, and a second outlet. The first inlet and the second inlet are adapted to receive a fluid. The contoured valve is disposed in the housing adjacent the first inlet. The contoured valve is rotatable about an axis from a first positional limit to a second positional limit, and to a plurality of positions therebetween. Fuel cell systems having the three-way diverter assembly for regulating temperature and humidity of a fuel cell stack are also provided.