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
Aramis tries to increase his power and do his friend, Fouquet, a favor and swap out a hostile King of France for a more amenable version.
Act II
Fouquet saves the real king but gets arrested anyway. Porthos and Aramis become rebels hunted by D'Artagnan.
Act III
Porthos dies. Athos dies. D'Artagnan dies. Aramis retreats to a Spanish estate.