electrical - Dryer Outlet - 4 prong to 3 prong - Home ...
They are "exactly what they look like": the 3-prong outlet omits the ground, and the 4-prong outlet adds the ground. Any problem with the neutral wire will result in the dryer frame being energized at 120V. Touching that and anything grounded, like the washing machine right next to it, will shock you. It is illegal to convert a 4-prong to 3-prong.