"Becky, you HAVE to ride in the car you have to ride in, just like we do. That's the way big people do it. When the ride is done, you will need to tell Daddy what a wonderful driver he is."
Then follow through, even though she protests. There could be a bit of power-gaming in this.
That said, three-year-olds don't have their imaginative life and their real life sorted out yet. It's amazing what goes on in their little minds. This is the age when they sometimes wonder if they can go down that little bitty bathtub drain - or if something can come up out of it! (Don't mention this out loud.) When you don't have to go anywhere, maybe you can make up a story with your daughter about a big, FRIENDLY car.