The three card monte game itself is very simple; a dealer places three cards face down on a table (often a cardboard box that can be quickly taken away), shows that one of the cards is the Queen of Spades, and then rearranges the cards quickly to confuse the player about which card is which. If the player correctly identifies the Queen of Spades, he wins an amount equal to the stake he bets; otherwise, he loses his stake. When the mark arrives at the three card monte game, he is likely to see a number of other players winning and losing money at the game. These are shills, confederates of the dealer who pretend to play so as to give the illusion of a straight gambling game. The mark is likely to notice that he can follow the Queen more easily than the shills seem to be able to, which sets him up to believe that he can beat the game.
To bring the mark in fully, a roper or outside man (as opposed to the dealer, the inside man) will approach the mark and suggest a way to turn even this easy win into a sure thing, a gamble that cannot be lost. He demands to examine the cards, and while handling them puts a subtle bend, or crimp, in one corner of the Queen of Spades. This makes the Queen especially easy to identify, and the mark is encouraged to win a small amount by finding the Queen using the crease. Often the outside man will put up the stake for this first bet, so that the mark doesn't have to risk any of his own money. The outside man will then suggest that he and the mark together bet all the money they are carrying, including the winnings from the first bet, on the next play.
The greedy mark will stake everything he has on the next bet, supported by the confidence of the outside man that they can't lose; but when he turns over the creased card, he finds that the dealer has removed the crease from the Queen and put it on a losing card. The mark has lost his bet, along with any part of the final stake put up by the outside man.
In the last stage of the con, the mark is cooled off by the outside man who curses the failure of the trick and the money he claims to have lost along with the mark. A particularly gullible mark may at this point go off to find more money to gamble on a second attempt, but even an ordinarily gullible mark may not realize that he has been swindled, or (if the outside man's share of the stake was large) may even feel that he has betrayed the outside man's confidence in his ability to win the game. After the mark leaves, the inside man returns the outside man's stake and a new mark is found.
Notes on Three Card Monte, by Whit Haydn
Catch 33: Three Card Monte, by Lee Asher available at http://www.leeasher.com