There are three caskets of gold, silver, and lead. This gimmick of choosing a casket to choose a suitor was an old folktale, and Shakespeare likely took it from the medieval Gesta Romanorum, which had been translated into English in 1577. In the play, as is done in the book, Portia's father left behind three caskets to symbolize three different kinds of love. There's also some serious paternalism caught up in the game - Portia must marry whomever chooses the correct casket, regardless of her own feelings or preferences. Nerissa points out that Portia's father was a wise man - his game is likely carefully devised so that only one who truly loves Portia can have her. Unlike many other challenges in the play, the casket game relies on nothing but merit for judgment. Every man - regardless of race, creed, or country - has a shot at her hand. The suitors' own wisdom, and not any prejudice against them, will decide their fate. This democratic judgment is set up in direct contrast with much of the rest of the play, where prejudice plays an influential role in how people are treated. It's more proof that Belmont is an idealized form of Venice - while all sorts of men may interact in Venice, only in Belmont can they interact on an equal playing field. Portia doesn't need a choice, because her father's judgment, and the men's own wisdom, will reveal what symbolizes true love to them. The man who chooses correctly chooses true love, so Portia wins when she wins him.