The trick to any good scam (especially one where money is at stake) is misdirection. This holds true for three-card monty.

Three-card monty is a gambling game in which three cards (cupped for easier movement) are placed face-down on the table. The marks (another term for "sucker") are shown where the queen is. The cards are then moved around in a quick fashion. The marks are then invited to place a bet on which card the queen is. Only one bet is allowed, and it is here that the scam comes into play.

If a legitimate mark places a bet on the wrong card, he is shown the card he bet on, and his money is taken. If a mark is about to lay down his money on the correct card, a ringer in the crowd quickly jumps in and bets on one of the other cards (the ringer and the person running the game of course have worked this out ahead of time). Thus, no legitimate mark can ever win any money except at the ringer's discretion.

The scam is so effective because the marks all think the scam is in the cards.