What exactly does this mean?
arrive Zagreb Sept. 13
Arrive what time, from where, by what means? Will you be jet-lagged coming off a 10-hour redeye flight from California at 4 pm, or fresh from a short drive at 9 am?
The drives you propose are not terribly difficult, and won't take all day. But they probably will take most of the day (when you factor in getting started, finding your way, putting the car away upon arrival and getting oriented). So, Day 1 is mostly consumed by driving - unless you roar off from Zagreb early and fast, you won't reach Rovinj until sometime in the afternoon. Day 2, you have a full day there, and you have just arrived - I'd spend it in Rovinj - why head off to explore other towns? Day 3...hmmm, you're going to drive from Rovinj to Plitvice, then drive onward and end the day in Zagreb? Don't you want to spend any time in the park at Plitvice, rather than just seeing the entrance sign as you drive by? As I recall, when I drove from Rovinj to Plitvice, we arrived in Plitvice around 3 or 4 pm. Of course, it depends on what time you depart Rovinj and how fast you drive, but it's going to take you more than a few hours to get there.
Bottom line - it looks like your plan is to "do" Zagreb, Rovinj and Plitvice in just 3 days. I don't think that's very realistic. Yes, you can get yourself to those places in 3 days, but you're not going to have a great experience in them. Plitvice deserves more than an hour or two, and Rovinj is perfectly suited to slowing down and relaxing. Can't help you with Zagreb.
I'd look to see if you can add another day or two if possible. Give Rovinj a full day and two nights, give Plitvice a night and most of a day.