How do I convert a 4 wire dryer cord to 3 wires?

See here:
Self Help and More - Dryer Circuit Wiring and Hookup (http://www.selfhelpandmore.com/switch-outlet/dryer-circuit-wiring-and-hookup.php)

Please read your instructions that came with the dryer.

They may mandate a 4 wire connection which would mean that the frame has to be grounded to a receptacle post.