A 3-way valve has, no surprise here, 3 ways in which a fluid can flow. This is why 3-way control valves are most used for diverting or mixing fluids. A fluid or flow enters the valve in port A and port B and leaves the valve, completely regulated through port C. Or a certain flow enters the valve at port A and flows out through port B and uses port C as an overpressure port.
3-way control valves come in various materials such as steel, steel cast iron, dunctile iron, aluminium or bronze. The choice of material depends mostly on the type of use of the valve. What is the temperature of the fluid, what is the pressure and where is the 3-way valve placed.