Jacqueline, the last time I priced one-way car rentals within Croatia, it cost somewhat more for short rentals (just a few days). The last time I rented (2009, via Auto Europe), there was a minimum 4 day rental period to avoid an extra cost, to rent in Zagreb and drop in Dubrovnik. Perhaps that's changed - but keep that in mind; it may cost a bit more for renting a car only 2-3 days.
Instead of saying near Plitvice, you might stay IN Plitvice. There are several hotels right in the park. They are slightly overpriced for what you get, but they are super convenient. Moreover, if you arrive later in the day from Rovinj, you could see the park first thing the next morning - arriving before big tour groups arrive is highly recommended - then head off to Split. You could even see the park part of two days, like I did. If you stay at a park hotel, you can walk right into the park, and they will stamp your ticket from the previous day at the hotel front desk so you can avoid paying a park entry fee the second day (at least that's how it was in 2009).
I'm not sure how two nights in Hvar AND Korcula (two for both?) would work. The daily catamaran service, if it's on the same schedule as it was in 2009, leaves Split late in the afternoon or early evening and stops in Hvar then on to Korcula. It may not be practical to try to see both with only two nights. You might pick one or the other, and Korcula is probably going to be easier. (I didn't visit Hvar, only Korcula).
Whether it's hard to get from Korcula to Dubrovnik by ferry depends on the time of year. In the summer, I believe there is daily catamaran service, but not off season - then you'd be limited to a ferry once or twice a week going up and down the coast, so you'd have to time it right with your departure day to Dubrovnik. Otherwise, I think there's a daily bus early from Korcula Town to Dubrovnik all year. On Sunday(?) there may even be two buses.
Another option would be to keep the car longer and drive from Plitvice to Split, then to Korcula via ferry. If you catch the daily car ferry right from Split on the right day you could do that, or you could drive south from Split and take two short ferries across the peninsula via Orebić (look on a Google map of Croatia). Then after Korcula you could take the ferry back to the mainland in the same way and drop the car in Dubrovnik. (I'm not aware of a similar path via car to Hvar - maybe there is.) Having the car adds some extra cost and inconvenience if you have to park it, but it gets you off of the rigid ferry/catamaran/bus schedules that can be inconvenient especially outside of summer.
Flying to Zagreb might work fine in your case. Croatia Airlines might have direct flight out of Dubrovnik to western Europe without a connection in Zagreb, maybe just once or twice a week, if you can time it right - then you could skip the connection in Zagreb. I had a direct flight on Croatia Airlines from Split to Paris; unfortunately it was canceled at the last minute and I had to take a (very early!) connecting flight via Zagreb anyway. But if there's a chance to avoid the stop in Zagreb - unless you really want a night there - maybe best to try and put that extra night somewhere else.