the most common reason for this is a foreign body usually food. your three-year old for some reason decided it was a good idea to put a green bean pea or? in the nose. if there is a bad smell and a nasty drainage from just one-side this is the most likely reason. nasal foreign bodies can be challenging to remove so it may be a good idea to allow the medical provider to do it for you.
3 year olds nose has a bad smell coming fron it