I have three red spots on the outside of the thigh. What could it be?

Many things. There are different types of rashes, and it's hard to know how "three red spots" actually appears. It could be bug bites. Or it could be an allergic reaction to something you touched. Or it could be a condition known as eczema can appear as red, itchy, dry patches. Or something else. Hard to know without seeing it and knowing more history. Best to see a doctor who can evaluate more thoroughly.