3 yr old saying it hurts when she urinates. has mucousy discharge. uti or yeast? how do they test for those?

Could be either: She could have one of the two things or both. Only way to know for she is to allow your provider to inspect the area and collect a urine for urinalysis and culture.

Children & Pain: It sure sounds like she needs to see her pediatrician. The doctor will do an examination and maybe collect some urine to test for reasons for infection or pain.