On a three wire configuration, the ground and the negative are tied together at the center position, using the
dryer ground strap. Following is a description of the difference between three and four wire. Hope it helps you.
These two
pictures illustrate the power
wiring on a the terminal of an
electric dryer. The one on top/left is the old-style three-wire configuration. Most people have this type in their homes. New
code changes, though, require that
dryers now have a four-wire cord, shown on the bottom/right.
Besides the number of wires in each cord, there are two important things to notice. First, in the four-wire configuration, notice that the dryer's grounding strap is folded back on itself. The whole point of the four-wire cord is to separate the ground from the neutral. The green wire (the "new" extra wire in the four-wire cord) is attached to the dryer cabinet. In the three-wire configuration, the grounding strap is left intact and the neutral and ground are tied together.
Sure hope this helps you find a resolution to your delimma! Best wishes.