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.
It's a little tricky to break this one into three acts, since there are so many subplots and schemes going on, and lots of the effects aren't experienced until a long time after something happens. Plus, it's already divided into five acts, and you know from fractions that's it's hard to split five into three. But we here at Shmoop are not fainthearted, so we'll give it a try.
Act I
Act 1 lasts pretty much from the beginning of the play to the moment the Duke (the old one) is killed. Up until then, Vindice could still turn back from his revenge plan. Lots of other things would still go haywire, but he could walk away from it all and continue his quiet life in the country, brooding on the skull of his fiancée. That dude could use a good psychiatrist.
Act II
Act II would likely go until the beginning of the masque at Lussurioso's "I'm a Duke Now" party. Everything's pretty keyed up, what with all the potential murders, aspiring dukes, and revenge plots floating around.
Act III
Act III is pretty much from the beginning of the masque to the end. The remaining characters meet some sort of end, whether death or glory, in quick succession. Then the play ends, with Antonio taking on the unenviable task of straightening out the dukedom and making everything better. At least he seems like a reasonable man for the job.