My 3 year old has a red patch on the iris of the eye...What canthat be?

Blood in eye. If you see a red patch on the colored part of the eye, the iris, take the child to an ophthalmologist for evaluation.
Red iris patch. A red spot on the iris of the eye can be many issues. Vacular, inflammatory, neoplastic, and other lesions can present in this manner. At this age, it is not likely neoplastic in nature but it needs further evaluation by a pediatric ophthalmologist.