Bassanio, who eventually gets to marry the fair Portia of Belmont, is from Venice (Italy). The play opens with a scene between him and Antonio, a wealthy Venetian merchant, in which Bassanio describes his spendthrift habits and asks for financial help from Antonio to woo Portia. Because of Bassanio's detailed knowledge of Venice and seemingly long-term relationships with other Venetians, it is logical to assume that he is a Venetian.
The Prince of Morocco is from Morocco; he is described as a Moor, and most likely would not have a royal position in that region if he were not a native to it.
Arragon is from Spain. Shakespeare wittingly makes him the most unlikeable suitor because of England's consistently untenable relationship with Spain. This is also the reason that Arragon chooses the worst casket and ends up with the image of an idiot instead of Portia's hand in marriage.