My 3 year old son complains about his penis being sore when he has an erection.no abnormalities on his penis.

Pain vs. erections: Depends on how well a 3 year-old can communicate penile sensations. If he and parents have talked many times about a penis being soft and just hanging there, then hard and sticking out, then soft again for no apparent reason; and that this normal penis behavior happens to all boys; then new pain complaints may be real. Otherwise, young boys may say "pain" when they first become aware of erections.