Robin, one night will be enough for Plitvice, as long as it's the night before you go into the park. It is extremely popular, and by mid-morning it is slammed with day-trippers, to the point that your enjoyment of the environment will be severely diminished. You want to get into the park when it opens so you have some blessedly crowd-free time on the walkways. If you arrive by mid-afternoon the day before, it might work to do part of your visit then, because a lot of the day-trippers will have departed. It doesn't take quite a whole day to make the longest marked circuit of the park, though if you're an inveterate photographer, that will take extra time. If you get the truly-essential early start, you should be able to move on to Rovinj the same day if you have a car.
The part of your itinerary where a car would be most helpful is probably Split-Plitvice-Rovinj-Zagreb. Plitvice has bus service from both Split and Zagreb, but nothing direct to Istria, and a car will free you from the tyranny of bus schedules. Your whole trip can be done by bus, however, if you decide not to fool with a car. You'll just not be quite as productive.
Istria without a car makes it difficult to see the two interior medieval towns I especially enjoyed, Motovun and Groznjan. Bus schedules are such that you can get to only one of them per day, and they're quite small, so you'd likely want to knock them both off on the same day. Other nice places to see are Porec and the tiny Vrsar. I haven't been to Pula for the Roman ruins; others on the forum seem to be sort of "meh" about it. As you can see from the number of places I've mentioned, 3 nights will be short for Istria. I think 4 would be better.
I like Zagreb a lot and think it's really good not to visit Croatia and see nothing but coastline and islands. Zagreb has a very nice bi-level historic district and a lot of interesting museums. Two of my favorites were the Naïve Art Museum (quite small, but with a really good collection) and the Museum of Broken Relationships.
If you use a rental car in the south, be sure you're authorized to cross Bosnia-Hercegovina to get to Dubrovnik; if you want to take the car to Mostar, that needs to be cleared ahead of time as well.
