Knee pain in three-year-olds may be related to synovitis, trauma, and juvenile idiopathic arthritis. 1. Synovitis: when the synovial membrane is infected or irritated, it may lead to synovitis, and the inflammation will cause swelling, distension and pain in the knee joint. 2. Trauma: It may occur when a child falls with uneven force while walking or descending stairs, resulting in trauma to the knee, which can also cause knee pain in children. 3. Juvenile idiopathic arthritis: a common connective tissue disorder in childhood. It may be related to genetics, infection, and immune response. It shows high fever and joint pain, and the knee is the most commonly affected joint. Knee pain in a three-year-old child can also be seen in other causes, it is recommended to consult a doctor in a timely manner, follow the doctor’s instructions for relevant examinations, with the help of the doctor to clarify the cause, and targeted treatment or treatment.