A Jandy (Pentair, Waterway, Hayward, Intermatic, etc.) 3-way valve is a valve in the shape of a "T." One of the legs is used as the main port (whether suction at a pump or supply in a return line) and a diverter is adjusted by a handle or motor to dertermine where the water is coming from (main-drain, suction-cleaner, spa, auxiliary suction port, etc.) or where it is going (spa, water feature, non-booster pressure pool cleaner, drain-line, etc.) using one or both of the other two ports. You only need one (or more) if you have to adjust the water flow in a manner described above.