I don't see a 3-digit # on the back of my Plat charge card (expires 2005)...I believe Amex uses the 4-digit number on the front for verification.

The 3-digit number is used on Visa and MC cards for card verification purposes for mail/phone/e-comm transactions. The number is derived from an algorithm using your account number, plus other account data, to ensure that in a card-not-present transaction, the card is valid. If someone gets hold of your account number (e.g. stolen statement, receipt, etc), they cannot provide this number.