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
In keeping with tradition, Act I begins at the beginning, as Jack and his mother drive across country to Utah. It ends at the conclusion of Part 2, with Jack's mom agreeing to marry Dwight and the two moving up to Chinook.
Act II
Act II comprises most of the book, from the beginning of Part 3 until the end of Part 5… call it Part 5, Chapter 5. Dwight beats on Jack, Jack rages back and they go at in until Part 5, Chapter 6 where it all comes to a head.
Act III
Act III lands in our laps fairly quickly because—while we can sense the build-up—there's no sign that this incident will be the one that breaks the floodgates. But break it does, from Part 5, Chapter 6 to the end of the book, detailing Jack's escape from Dwight and immediate mop-up operations.