What will happen to exposed 35mm film left in a car in summer heat for 3 weeks?

Film is perishable! It spoils when storage conditions include high temperatures and the latent image (undeveloped) degrades when their is a prolonged period between exposure and processing. That being said, amateur films are designed to be more robust than those slated for professional use. What I am saying is, your film will still display an image. The color balance will be weird but --- when the film is printed, the automation of the printing machine will recognize this off-color and attempt to mitigate by applying correction factors. Nevertheless, the results will be substandard. When you pickup the film from the processing lab, don't leave without looking at the content of the package. Ok to complain if the color is bad. A reputable lab will re-print with manual corrections applied at no charge. If they don't --- next time choose a different shop.

The colors will probably shift a bit more than if you had stored it in proper conditions for those three weeks. As Alan answers in great detail, the shift should not be great and proper processing can correct most of the difference.

Leaving the film in the trunk will have caused significantly less color shift than if you had left the film in the passenger compartment while parked in direct sunlight for three weeks.

You didn't say what part of the world you are in, so we can only speculate. Where I live in a subtropical climate, the temperatures routinely reach 90°F (32°C) and above in the summer. If my car is parked in direct sunlight, the passenger compartment can get to 140°F (60°C) or more, while my trunk tops out at about 105°F (40°C). If the car is parked in the shade, the temperature in both the passenger compartment and the trunk stays about the same as the ambient temperature.

I just developed some film that was left in my car for both winter and summer, I did not notice a big difference. Film Kodak Max 800.