Although Jerry Seinfeld has been relentlessly plugging his family-oriented animated flick Bee Movie, and although it is opening in 3,500 theaters and about twice that number of screens, box-office prognosticators figure that An American Gangster will lead the box office this weekend. They are also predicting that a third film, Martian Child, starring John Cusack, will flop, earning perhaps less than $5 million.