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
At the start of The House of the Seven Gables, Hepzibah and Clifford are living in hell. Hepzibah has walled herself up in her family home, living in cold, dilapidated gentility, and Clifford has been rotting in prison for 30 years. But there is a ray of hope: Clifford gets to go home to his family, Hepzibah starts interacting with society more by opening a store, and young Phoebe Pyncheon arrives from the countryside to improve life at the House of the Seven Gables.
Act II
The second act is the part of the story where everything seems as far away as possible from a conclusion. Because the plot starts with Hepzibah and Clifford's misery, we guess that the main conflict of the novel is going to be their finding a better life. This better life seems pretty darn distant around Chapter 15. Phoebe has gone on vacation and everyone is waiting desperately for her to get back. Judge Jaffrey Pyncheon arrives threatens to commit Clifford to an asylum if he doesn't produce some completely imaginary source of wealth. All of the little joys that Hepzibah and Clifford have been building up with Phoebe and the Pyncheon garden seem on the verge of destruction.
Act III
But then Judge Pyncheon dies of a sudden stroke. His death gives Mr. Holgrave, the lodger, the extra boost he needs to tell Phoebe he loves her. And the evidence of Judge Pyncheon's natural death clears Clifford of the (equally natural) death of his uncle 30 years before. Judge Pyncheon's fortune goes to the remaining Pyncheons, Phoebe, Clifford, and Hepzibah, and they all decide to move to Judge Pyncheon's lovely house in the countryside.