How often should she eat?

My daughter is three months old and eats every three hours, 4-5oz usually, sometimes 2-3oz. Her pediatrician thinks she's eating too much and too often and needs to be eating every four to six hours instead but I don't know how I'm supposed to change that because I sure as hell am not letting her sit and cry because she's hungry. When she's hungry she's hungry and I'm feeding her on demand, not on a schedule. How often and how much should she be eating at three months?