Born in the mid 1800's, the Three Card Monte hasn't change much since, and although it is renowned to be a hustle, people still fall for it. It's simple: the card man has three cards, a red queen and two black cards. He tosses the three cards on the table, shuffles them and asks you to find the lady. In fact it's so simple that it seems there is no way you can lose! Don't be fooled: the victime never wins money.
The Monte has several steps: first, the victim is attracted by the game, often because a beautiful lady (the hook) pushed him in, or maybe just out of curiosity. If the victim was with his girlfriend, it won't take long that they'll be split up by the crowd, so the girl doesn't interfere with the bets.
Then, the straight toss: the card man tosses the cards slowly, so that it's obvious where the queen is. At this point, the victim doesn't want to bet, but he'll be following the queen to see if he's got what it takes. Another guy puts money on the card, and wins. The victim takes confidence, because it was the card he would have bet on, too. Also, the card man may offer a free game to the victim, no money involved, so that he can see how easy that game is. Of course, the winner is a member of the Monte team (the shill) : remember, nobody ever wins. The victim can also be forced closer to the game to lay a bet for another player (maybe a beautiful blonde?)... yet another shill.
Now, the hype. After a couple of straight tosses, the victim is ready to bet; the card man tosses the cards. However, this time the toss is slightly different, although it looked the same. The queen isn't where the victim thinks it is, and he loses the bet. Magicians will often swap the winning card for a THIRD losing card. This is never done by con artists despite being depicted this way in films, tv and books.
If, by any luck, the victime bets on the queen, then he'll be outbid by a shill who'll bet on a different card. Gotta take the highest bet... the victim will never be allowed to place a winning bet and get the money. Plus, this will infuriate the victim, as he saw he had chosen the right card, and he'll put more money on the next round -- this time on the wrong card.
If the victim becomes agressive, then the Heavy will bump in, and you don't want that. This guy is a professionnal who hurt people for a living.
If the police gets too close, the wall-men will shout "Slide!" and the game breaks up. Everyone vanished before you know it.
And there are many more variations of the game. So let's resume: the card man, the hook, shills, the heavy, wall-men... in a game of Monte, the victim might be the only person involved who isn't a member of the team! There is no such thing as an honest game of Monte: stay away from it. Whatever others may tell you, scammers will never let you win a couple of bets. Whenever you see anybody win at Monte, he's part of the team.
Of course, if the victim doesn't bet and only wants to watch, the game is a massive distraction to allow pickpockets to crowd in and pick the victim clean.
(ref. How to Cheat at Everything, by Simon Lovell, Running Press)