What is the multi-layered significance of the 3-card monte hustle in "Topdog/Underdog"?

The 3-card monte hustle is significant because it reinforces the concepts of chance/ risk and the volatility of life.

The card game is central to the story; it highlights how Booth and Lincoln choose to live their lives. Both gamble on more than a superficial level; they also pit their wits against each other in the game of life. The card game reinforces the concept of life as a game, where one must "hustle" in order to get ahead. In the play, risk-taking behavior is seen to be a multi-generational practice. Years ago, Booth and Lincoln's mother had a risky, adulterous relationship with the "Thursday" man and gambled that she would never be discovered.

When Booth caught her in flagrante delicto (or in the midst of a tryst with the "Thursday" man), she paid him off with five hundred dollars. At least, this is how Booth sees it; he suggests that his inheritance from his mother is actually "hush" money. Not to be bested, Lincoln proclaims that their father also gave him ten fifty dollar notes, which amounts to five hundred dollars.

Both brothers are willing to risk their reputations and lives for different things. While Booth wants to use 3-card monte to "rake in the money," Lincoln gambles on doing Abraham Lincoln impersonations to earn an honest living. Additionally, Booth steals from different shops in order to procure alcohol, new suits, and pornographic magazines. He looks at Lincoln as his ticket to greater wealth. Because Lincoln is a gifted 3-card monte player and has the skills to make an obscene amount of money, Booth pressures him to work together as a team.

Lincoln refuses. Booth is willing to risk everything to bring in more money; his object is to make sure he wins Grace. In the end, Grace resists returning to Booth because of her dissatisfaction with his ability to make a decent living. Discouraged by this development, Booth kills her and later challenges Lincoln to a high-stakes game. Each bets his individual inheritance against the other. As the superior player, Lincoln wins, and Booth is distraught by this. In a last act of desperation, he kills his brother in order to save his inheritance.

So, the 3-card monte hustle is significant in that it highlights the tragedy of multi-generational risk-taking, reinforces the volatile nature of life, and emphasizes the intrinsically risky nature of the game.