The Fat Boys, the rotund rap trio, have sued Miller Brewing Co. for $5 million over a TV commercial in which comedian Joe Piscopo and three heavyweight partners rap a song touting Miller beer. The federal suit filed Monday in New York claims the ad takes advantage of their platinum-selling record success and infringes on the Fat Boys' trademark. They also claim the commercial's music "contains a melody copied from (nd) derived from" the Fat Boys' music. The portly plaintiffs are Mark Morales (etter known as "Prince Markie Dee"), Damon Wimbley (ka "Kool Rock Ski") and Darren Robinson (`The Human Beat Box"), along with the group's record company and music publisher. They also seek the withdrawal of the commercial. The ad agency behind the commercial, Backer & Spielvogel Inc. of Manhattan, declined to comment. A Miller spokesman was not available at the company's Wisconsin headquarters.