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 book until Sir Thomas arrives home and breaks up the theatrical production.
Immediately following the end of the theater until Fanny refuses Henry and is subsequently sent off to Portsmouth.
Fanny's time in Portsmouth until the end of the novel.