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.
This one's pretty simple.
Act I
Readers, meet our four (plus one) main characters: Biff, Mick, Jake, and Copeland. And of course, they're main squeeze, Singer.
Act II
This is pretty much the bulk of the plot. We'd suggest, you know, reading the book.
Act III
Here we get a sort of multi-chapter epilogue, with quick glimpses into the future of our gang's lives.