yep... you just have an extra wire in there. Don't need to use it at all. There should be two "hot" wires that each will measure 115V to ground, or 230V between the two. Those, and one ground wire should be all you need.
The color wire that you use (other than the green) is strictly up to you, depending on the plug configuration, of course.
It sounds like you bought a cord designed for three-phase power. (4 conductor) Your pot should be single phase. (3 conductor)