Naples, just like Venice, is where I would suggest longer stays. Unlike Venice, accommodations is not costly. I love Naples – Amazing accommodation; Prices of food reasonable; Friendly locals; Beautiful castles; and amazing markets.
Getting there: There are trains and buses from Fiumicino Airport to Naples. From Rome, trains from Roma Termini station or Roma Tiburtina station to Napoli Centrale. Local trains from Pompeii to Naples – Visit https://www.eavsrl.it/web/ for Train schedule and Pricing.
Suggestion: Depending on what you want to see. 3 days stay for me was not enough. I had an awesome Airbnb accommodation. Amazing Host and his humble family.