Prone means lying face down.
- The old man enjoys sleeping in the prone position.
Prostate refers to the male gland.
- The doctor recommends having a yearly prostate exam.
Prostrate also means lying face down (often after an accident) or overcome with grief.
- The football player lies prostrate after the tackle.
- Understandably, she was prostrate with much grief.
Supine means lying face up.
- The beach is full of sun worshippers, supine on the sand.