Exposed color film can be affected by prolonged heat, because film is perishable and the latent image slowly degrades before processing. After three weeks in a hot car trunk, you should still get images, but expect some loss in quality.

The most likely issue is mild color shift or odd color balance. Consumer films and disposable-camera film are generally fairly robust, so the damage is often less severe than people fear. A trunk is also usually cooler than the passenger cabin in direct sun, which helps.

In many cases, standard lab printing/scanning can correct much of the color shift automatically, and a good lab may be able to improve results further with manual corrections. Even so, the final images may still be somewhat substandard compared with properly stored film.

So: you’ll probably still get usable photos, but with a higher chance of off colors and reduced image quality. Have the film processed soon, and if the first prints/scans look poor, ask the lab whether they can reprint or rescan with manual color correction.