Bad breath in a 3 year old child can be caused by Helicobacter pylori infection, but it can also be caused by indigestion and poor oral hygiene. 1. Helicobacter pylori: After a child is invaded by Helicobacter pylori, it may cause the child to have bad breath. 2. Indigestion: some children have weak digestive ability, if they eat more high-fat and high-protein things, it may cause intestinal peristalsis slower, prompting a large amount of food stagnation in the stomach and intestines excessive fermentation caused by bad breath phenomenon. 3. poor oral hygiene: some children may be due to long-term oral hygiene care, resulting in dental caries, food residues in the mouth excessive accumulation will cause bad breath phenomenon. 3-year-old children may also have other causes of bad breath, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, to give targeted treatment or treatment.