Use a Philips head screwdriver to remove the small metal plate on the back of the dryer that covers the terminal block. A dryer like a Kenmore,may have one 1/4-inch bolt holding the plate over the terminal block. Use a 1/4-inch nutdriver to remove the bolt.
Use a Philips or a flathead screwdriver to remove the dryer cord retainer clip. Some manufacturers use flathead screws.
Remove the 4-prong dryer cord from the terminal block by loosening the nuts. Some dryer manufactures like Kenmore may use flathead screws that bolt into the terminal block.
Install the 3-prong dryer cord through the cord retainer clip and to the terminal block. Attach the center wire to the center post of the terminal block first and tighten. The center wire must be on the center post. Attach the outer wires and tighten. Make sure the wires are secure, and make contact with the terminal post.
Secure the retainer clip, and bolt the cover plate back over the terminal block. Make sure your dryer venting is attached securely. Push the dryer into it's spot, and plug the cord into the receptacle.