Julia Roberts, Tony winner? It's not as impossible as you might think. Next March, the Oscar winner will follow Nicole Kidman, Hugh Jackman, and Denzel Washington as the latest movie star to hit Broadway when she debuts in a 12-week revival of Richard Greenberg's 1997 play Three Days of Rain. Introduced to the piece by its director, Joe Mantello (Wicked), Roberts, 37, staged a reading in late June with Greenberg, Mantello, and producer Marc Platt in L.A. ''It's beautifully written, and now that Julia's a mom, I think its notion of motherhood spoke to her,'' says Platt, adding that the two male leads are uncast. With the three actors taking on roles equal in size, Roberts, who'll play both an alcoholic woman and her daughter, won't carry the show on her own. That and Rain's low profile could help her debut go down smoothly with the theater press (its status as a box office success is virtually a given). Says Greenberg: ''What impresses me is that it's such good casting. It's not simply that the biggest star in the world is doing your play, but it fits her so well.''