There are many reasons for mouth ulcers in a 3 year old baby, such as vitamin B, vitamin C deficiency, folic acid deficiency, and other nutrients. There are also reasons such as dehydration, dryness, and fire in the child’s body that may cause mouth ulcers to appear inside the baby’s mouth. It is not advisable to take too much medication when the child is too young, and mouth ulcers can generally be cured in about 5-7 days. At the same time, in order to speed up the healing process, it is recommended to give your baby dietary therapy and lifestyle changes, such as eating more tomatoes, oranges, watermelon and other fresh fruits and vegetables to replenish the various trace elements in your baby’s body. In addition, drink more water and do not let the baby in too dry environment, can promote the mouth ulcer to heal faster.