Person: I broke my arm in three places.
Doctor: Well, don't go to those three places then.
Person: I broke my arm in three places.
Doctor: Well, don't go to those three places then.
The humor comes from the doctor taking the patient's statement about breaking their arm in "three places" literally, as if the locations themselves caused the injury, rather than the injury happening at those locations. It's a silly way to avoid medical responsibility.