I saw rick's greek 3 island cruise video but either he didn't mention the cruise ship or I missed it somehow.
You'd think the cruise ship would be out of Athens, however there's no 3 day cruise out of there. I would assume ferries would be the best way to go from island to island in a short amount of time.
We once went to a number of Mediterranean and Greek islands out of Rome on a 12 day cruise. It was a great trip.
Suggest you go in cruisecritics.com as that is the best cruise forum period .
We went on a cruise out of Athens that we added to our bus tour package Louis Cristal that has since been bought by another company - it was enjoyable- not a huge ship. The website is https://americas.celestyalcruises.com/en
Rick was on Celebrity, and I believe the ship was Reflection. It wasn't 3 days, I think it was at least a week. I recall from his blog that his itinerary included Malta, among other ports. Lots of posts about it on his blog.