Feed on demand or schedule

My daughter is a little over a month old & ive been making sure that she never goes over 3 hours without eating. Sometimes i have to wake her up to eat, should i just let her sleep? I asked her pediatrician at her 2 week appointment if i could let her sleep cause she was back to birth weight & he told me no because babies cant regulate their blood sugar.

Anyways, question is should i let her tell me when shes hungry or should i keep making sure she only goes 3 hours without eating?