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
Holly and the narrator meet and become friends, and he then finds himself quickly entrenched in her chaotic life. When Holly decides to leave New York with José, there is no going back for her or the narrator since their relationship is forever changed.
Act II
Holly gets arrested for being involved with Sally Tomato, and the neat and tidy life in Brazil that she imagined for herself suddenly becomes farthest from her reach. Neither Holly nor the narrator (or us for that matter) knows what will happen to her.
Act III
José leaves Holly, she leaves New York, and the narrator gets a postcard from her from South America. He becomes a successful writer and moves out of the brownstone but we're not really sure what happens to her.