For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
There's a lot of rebellion and rescuing going on here. Sure Jimmie needs saving, but Maggie needs it even more, so thank goodness Pete's here to save them both from imminent violence. But it's not all good. This act goes from the opening to when Pete reappears and Maggie takes a shine to him.
Act II
Thanks to Pete, Maggie finally glimpses life outside the four filthy walls of her Rum Alley tenement. The spectacles in her life no longer consist of neighbors staring at the violent dramas that spill out of the Johnson apartment into the halls. Maggie is enjoying all sorts of theatrical events with Pete, and this act covers the duration of their relationship.
Act III
Okay, so maybe Pete's not so classy after all, since he ditches Maggie for Nellie, his old flame. But while theirs was a short-lived romance, it ended about as badly as possible. This act runs from when Maggie is dumped by her man to her death at the end of the book. Hey, at least she doesn't ever have to see her tanked-up mother ever again.