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
Act I lasts until Inman decides to run away from the Confederate Army hospital and go home to Ada. That's the point where he's committed to going home and the plot really kicks off.
Act II
Phew! Act II goes on forever, really until the moment that Inman is shot by the Home Guard at the very end of the book proper. Talk about far from resolution. We don't even know if the hero is alive or dead until the Epilogue.
Act III
Wow, this epilogue works overtime. It's pretty much all of Act III, where everything is wrapped up. We finally learn what happened to Inman, Ada, Stobrod, and Ruby, and a few other characters too. Stick on that last piece of tape and tie a bow on it.