The following are possible causes of rhinitis in 3 year olds: 1. 3 year olds have an incomplete immune system and are susceptible to external viral and bacterial invasion at this time, resulting in rhinitis, which generally decreases with the age of the child and the probability of rhinitis occurring after a sound resistance, especially for acute rhinitis. 2. 3 year olds may be sensitive to some allergic substances due to their parents’ genetic factors.