J.W.
At 3 she's able to understand actions and consequences when they're described and then followed up promptly. So let her know that if she gets out of her seat, the car has to pull over and stop and it won't go as long as she's out of her seat. It's the law and the policeman would stop the car if he saw her and then Momma or Daddy would be in trouble, big trouble. So, when she gets out of her car seat, you'll need to follow-up and pull off the side of the road, where it's safe, and stop the care until she gets back in her seat. You can leave her at home with a sitter or with Dad while you and the baby go out to the store, etc. or maybe while you drive to the park for a fun time she'll be left at home because she can't mind you and stay in the seat.
It's very important that the consequence follow her getting out of her seat right away. Don't waffle or make excuses if you want to be successful. Also you didn't mention what prompts her to get out of her seat? Boredom? Wants a book or toy? Wants to sit by a window? You're stopped and she wants out first? You could find a way to keep books and a toy near her seat. If it's a window seat, move her seat to that window. Getting out to get out of the car first... sticker chart for every time she waits, so many successful trips, a treat.
Wish you lots of luck and patience!!!