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
Dorothea's marriage to Casaubon, his death, and her discovery of the codicil in his will forbidding her from marrying Will Ladislaw.
Act II
ends when Dorothea walks in on Will Ladislaw and Rosamond Lydgate and thinks they're having an affair.
Act III
Everything gets resolved – Rosamond explains it all to Dorothea and leaves Will free to marry Dorothea. After Dorothea gives up her fortune.