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
Act I corresponds to, you guessed it, Act I, in which we meet the characters and Polly marries Mac. Peachum goes berserk over the secret wedding, setting up his actions in Act II.
Act II
This is easy: Act II is… Act II. Mac the Knife has to run away from the coppers, but he gets caught anyway when his prostitute friends betray him. He is sent to jail, but escapes.
Act III
We could do this in our sleep. Act III = Act III. Mac is caught yet again and sent to be hanged, but saved at the last minute. Everything ends bizarrely happily.