3 port fuel pump

Third or small port is a return line to the fuel tank. This port helps solve fuel vapor lock. 

When you think about it, we really do not use very much fuel as we travel with our units. Therefore, there really is very little fuel flow through the fuel lines, basically stagnating and just sitting in the fuel line and pump. The fuel heats up at the pump and the lines leading to the pump near the engine. During this time, the fuel can vaporize and when it is in a gaseous state it simply pushes back and forth through the intake check valve of the fuel pump. The fuel pump cannot deliver sufficient fuel to the carburetor as required by the engine.

By keeping some fuel flowing through the fuel pump and low pressure fuel line, the temperature of the fuel is significantly lowered and vaporization is avoided.

Here are some numbers to help at the parts counter
Airtex 40736 Bosch 68622 Carter M6109
Ask for a Pump for: 74-77 Olds Toronado Broughm 455 with Air