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
The Dorrits are deep in debt and deep in the service of their father's pretentions to former gentility.
Act II
Pancks discovers that the Dorrits are actually fabulously wealthy and they go off to a completely new life of ease, comfort, and society.
Act III
Dorrit's investments go bust in Merdle's collapse. All of the old generation that stands in way dies (Mrs. Clennam and Dorrit), and Amy is free to marry the man of her dreams.