Sleep training and night feedings

Alejandra
I'm trying to sleep train my 3 month baby boy and I've had no success so far. Doctor says that he can go all night without feeding now that he's over 12 pounds but I'm having trouble implementing that. I understand that I don't have to wake him up to feed at all. However, lo doesn't sleep for more than 3 hours at a time and it takes hours to get him to back to sleep.
Since it takes so long to fall asleep and stay asleep, I wonder if it's because he's hungry because he wakes up so often. I'm up all night trying to get him to sleep and he won't, so I feed him because he's gone 3 or more hours since eating. Is that interfering with the sleep training?