If a three-year-old child is suspected of having Helicobacter pylori infection, he or she can be tested through a breath test, fecal Helicobacter pylori antigen test, and blood Helicobacter pylori antibody test. 1. Breath test: It is a non-invasive test. When fasting, take drugs containing carbon 13 or carbon 14 isotope, wait for a period of time, and then exhale into a breathalyzer, which can diagnose the infection by detecting the level of carbon 13 or carbon 14. 2. Fecal H. pylori antigen test: This is also a non-invasive test, in which feces are taken and then cultured and analyzed to assist in the diagnosis of H. pylori infection. 3. Blood H. pylori antibody: Blood of a three-year-old child is taken to detect the presence of antibodies against H. pylori, which is used to assist in diagnosis. Gastroscopy and stomach tissue biopsy are also available, but they are not easily tolerated by small children and are generally used with caution because of the damage caused by biopsy.