Help plan a 3 day trip of Croatia flying or taking a ferry from Italy

There are good flights from Rome, as well as other cities may work. Use www.skyscanner.com and in addition to city-to-city searches, you can also search to/from a whole country. Ferries leave mostly from Ancona, Pescara, or Bari; see www.aferry.com to search, or www.jadrolinija.hr is the largest Croatian ferry company. Even with cheap flights, it sounds to me like a bit too much hassle for only a 3-day stay.

When is he travelling?

Renting a car in Croatia (or many places in Europe) will be difficult or more expensive if aged under 25. Getting a cheap flight will be the best option, but these only go up in price and they have been available for months, so the best deals will have gone. Look at Easyjet and Ryanair for the best deals. This will be cheaper than a ferry.

With only 3 days, he is best just going to one place. I would suggest Split or Dubrovnik. There is plenty to see in both with day trips available such as Split to Trogir or Plitvice Lakes OR Dubrovnik to Korcula or Mostar. Buses are the way to get about (there are no trains in Dubrovnik).

Airbnbs are fine and plentiful - they vary much like anywhere else. Many Airbnbs have a minimum 3 night booking in high season (from mid June when the local school children start their holidays) and particularly on the islands.

He needs to google his actual flight dates re the flight to Paris. Split and Dubrovnik airports have good websites and available in English, which shows the carriers and destinations served. There are certainly flights from Split from memory, but nowhere near as many flights as to say the UK or Germany.

Pity he has only 3 days as it's a great country and I enjoy it more than Italy.

My advice is to stay longer than 3 days. With three days, you'll only be able to see a single city and surrounding areas, and there is so much to see in Croatia!

Croatia Airlines has a direct flights from Split and Dubrovnik to Paris, but they only fly on certain days of the week. From Zagreb, Air France and Croatia Airlines have direct flights to Paris every day. Of course, there are plenty of other options if they don't mind a flight with two legs.