The yellow discharge in the underwear of a three-year-old baby may be caused by urine residue or urinary tract infection, vaginitis in young children, etc. Different treatment measures can be taken according to the cause. Common causes and treatments: 1, urine residue: If the child has no remaining discomfort, it may be due to too little water, improper diet and other factors that cause fire, resulting in yellow urine, residue in the panties as yellow discharge. You can drink more warm water, adjust to a light diet, the symptoms can be gradually relieved; 2, urinary tract infection: the baby appears urinary tract infection, causing abnormal urine, will also lead to yellow urine, visible urethral orifice red, with painful urination, to carry out urinary system examination as well as urine routine examination to confirm the diagnosis. If a urinary tract infection is diagnosed, anti-infection treatment can be administered as prescribed by the doctor. 3. Vaginitis in young children: If a young child complains of itching in the vulva and yellow purulent discharge, consider that the child has vaginitis. It is advisable to go to the hospital for routine laboratory tests of the white belt to clarify the pathogens of infection and then follow the medical advice for anti-infection treatment. Note: Pay attention to hygiene and wash your vulva with water. Drink more water and eat less spicy and greasy food, as well as seafood, mutton and other hairy products, and eat more fresh fruits and vegetables to help relieve the symptoms.