When I was in Croatia (three weeks in May last year that also included Slovenia and Bosnia), I only had about half a day in Split. It turns out that was plenty of time to tour Diocletian's palace and wander around. I was not rushed at all and feel like I saw everything of interest in the area around the palace. I then caught a late afternoon catamaran to Hvar, where I spent two nights.
With three days, and assuming you'll want to spend a full day in Split, that leaves you two days for excursions. Because May is still early in the season, there aren't a lot of ferries, so coordinating trips to 2 different islands will not be easy. I'd recommend Hvar, which is the closed island, and probably easiest to do as a day trip.
The disappointment for me was that I didn't have time (or budget time more wisely) to get to the Ivan Mestrovic Gallery. I'm a big fan of his work. You might want to devote a few hours to that.
I spent the night before arriving in Split in Trogir. That would be worth an excursion. It's about a 30-45 minute drive, and I'm sure you could figure out how to get there by bus.
And Plitvice Lakes can be done as a (long) day trip. In my opinion, it is not to be missed.