3 months old baby leg bruises are common in the following cases: 1, because of trauma or accidental bump caused by the rupture of the subcutaneous blood vessels in the legs, thus presenting as bruising phenomenon, within 24h can be cold towel cold compress to play a role in stopping the bleeding, if not very serious do not need to make cold compress. After that, hot compress can be applied to promote blood absorption, most of them are not completely absorbed and recede in about 5-7 days. 2, blood disease, bruises on legs without obvious history of trauma, and the bruises will gradually have an increasing trend, when there is trauma blood is not easy to stop bleeding, and easy to have nosebleeds, etc., you need to go to the hospital in time to exclude the possibility of blood disease.