Dryers can be converted from three-prong operation to four-prong operation by replacing either the dryer cord or the wall outlet. Most professionals consider changing the cord to be the best method. Adapter cords are available at most home improvement stores, such as Lowe's or Home Depot.
Cords with three prongs are an older style commonly used before 1996, when the NEC code was changed, requiring all dryer plugs to have four prongs. The change was instituted to make dryers safer and prevent electrical shock. Older three-prong styles carry both the neutral and ground wires combined into one, whereas the four-prong plugs have the wires separated.