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
From the opening of the novel, when Bayard and Ringo make their map of Vicksburg, to Granny's death at the hands of Grumby, we get to our point of no return. Bayard, Ringo, and Uncle Buck have no choice but to track Granny's killer and get revenge.
Act II
Starting with the beginning of their hunt for Grumby and ending with the confusion of the pistol shots between Grumby and Bayard, in this section the characters are about as far as you can get from resolving things.
Act III
The boys nail Grumby's hide to the door of the compress, and the novel unwinds all the way to another revenge story, this one between Bayard and Ben Redmond after Ben murders Bayard's pop. Bayard's change in attitude about revenge during the time between Grumby's and Redmond's actions shows how much he has matured. Ta-da!