The causes of red and swollen gums in three-year-old babies are as follows:1, the crowns of the milk teeth are short, the neck ridge is prominent, the adjacency is poor, often embedded food fibers, stimulating the gingival papillae red, swollen, distended, and painful.2, long-term nibbling of hard textured food or toys, etc., can bruise the surface of the mucosa to cause bacterial infections, and obvious redness, swelling, and pain.3, upper and lower milk teeth for the deep overlay, deep cover, the incisal end of the lower anterior tooth will rub the upper front teeth on the tongue side of the gums, forming traumatic ulcers, causing swelling and pain. Gums, the formation of traumatic ulcers, triggering swelling and pain. 4, the formation of deep caries in the milk teeth can cause periapical periodontitis, a large amount of inflammatory secretion in the pulp cavity will penetrate out of the apical foramen, breaking through the bone wall, resulting in swelling, bulging gums, and sometimes the formation of fistulas or sinus tracts, leading to tooth loosening.