In A Traditional Theatre Format.
In most traditional plays or operas the story is split up into 3 acts: Discovery of the Conflict, Complications, and Resolution. How would you split up Psycho?
Act 1: Everything in Phoenix, Marion on the road, buying the car and evading the cop and arriving at the Bates Motel. Now this part is tricky. Does the act end when she arrives, or when Norman returns to house after peeping, after the shower, or does it go all the way until after Norman disposes of her car?
This is where Psycho is a different kind of story, since the main protagonist is already gone, and the main plot has switched.
Act 2: I believe it starts with Norman confronting Mother about the murder, then rushing to the motel, and disposing of the body. At this point it's no longer a story about a theft, but a murder mystery. The act runs through Sam and Lily meeting, all of Arbagost, then ends with meeting the Sheriff and his wife, and them informing us that Mrs. Bates has been dead for 10 years.
Act 3: Starts at the church where the Sheriff says he went out to the Motel and found nothing. From here Sam and Lily move toward the final resolution.