Jeremy Piven is not a cutthroat superagent. He's not married and he hasn't just been ousted from a high-powered Hollywood firm. In other words, he's nothing like Ari, the character he plays on HBO's "Entourage," which returns Sunday night for its third season. Piven does have plenty in common with the show, which follows up-and-coming actor Vince Chase and his entourage of New York pals as Ari and other Hollywood forces shape his future. As real-life actors, each cast member brings personal experiences to "Entourage," Piven says. "It's not a documentary, but there are certain energies out there in this...