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.
D’Artagnan arrives in Paris with not much more than the big dreams of becoming a Musketeer.
D’Artagnan becomes best friends with three Musketeers, and together the four of them have all sorts of glorious adventures.
D’Artagnan becomes a lieutenant in the Musketeers.