In order to install a 3-prong dryer cord, insert the cord into the installaion hole provided in the electrical connection access box in the rear of the electric dryer.
Unscew the three connection screws using a magnetic nut driver, so the screws don't fall down inside of the machine.
Connect the outer two wires (hot wires) to the outer two screws, obviously, one on each screw.
Now connect the center wire (neutral) to the center post. You'll notice the white machine wire is now the case ground connection, unlike the 4-prong cord connection in which the green wire from the cord makes the ground connection.
