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 begins with Sarty about to lie to the court about his father's barn burning activities. It ends, ironically, with Sarty's father accusing him of planning to tell the truth.
Act II
Act II begins with the Snopes family's arrival at the de Spain farm. It includes all the drama and funny business with the rug. It ends with Abner and Sarty running away from the sound of gunshots and the burning de Spain barn.
Act III
The final act begins with Sarty mourning the death (literal or figurative) of his father. It ends with him walking down the hill, toward the woods and the birds.