My daughter has a set of three red dry spots on her bottom. She's had it for about 4 weeks now. What to do?

Details about spots. Without knowing a number of other additional details, such as your daughter's age, size of the spots, how the spots are grouped (if at all), whether they are tender, raised above skin level etc, there is no way to guess what they might be or represent. My suggestion is to call your daughter's doctor, describe using your description and the items above and see what they recommend.