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.
From the beginning of the novel until Mr. Tulliver loses his lawsuit
The aftermath of the lawsuit until Mr. Tulliver’s death
From Maggie’s introduction to Stephen Guest until the flood and the death of Tom and Maggie