Three-Act Plot Analysis
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.
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.
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.