It is not at all unusual for little girls to have urinary tract infections. They are far more common in girls than boys because girls have a much shorter urethra, making it easier for bacteria to enter the bladder. A 3 year old is still learning to take care of her own bathroom needs, and may still have an occasional toilet accident or may not wipe herself properly. Bacteria in her underwear cam make its way into her urinary system. Urinary tract irritation can also be caused by scented chemicals used in bath water, like bubble bath.