Drooling of three-month-old babies is related to salivary glands that are not yet well developed, mouth ulcers, thrush, and so on. 1. Salivary glands are not yet well developed: usually the salivary glands of three-month-old babies are not yet well developed, so it is easy to drool, which is a normal phenomenon. If the three-month-old baby in addition to drooling, not accompanied by other uncomfortable symptoms, parents may not need to worry too much. 2. Mouth ulcers: the cause is still unclear, may be related to infection, oral mucosal injury, nutritional deficiency and other factors. When three-month-old babies suffer from mouth ulcers, ulcers will appear on the oral mucosa, accompanied by drooling, low-grade fever, localized lymph node enlargement and other symptoms. 3. Thrush: thrush will cause three-month-old babies drooling, but also easy to accompany the oral mucosal surface covered with white milk clot-like dots or small flakes, and irritability, crying, refusal to eat and other symptoms. Thrush is mainly caused by the over proliferation of Candida in the oropharynx, mostly seen in infants and young children, immunodeficiency and other special groups. Parents are advised to wipe their babies’ saliva with special tissues for infants and toddlers in a timely manner to improve their comfort level. If the baby has other symptoms, parents should take the baby to the doctor.