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Waking every 3 hours is normal. Completely and utterly normal. Far more babies still wake at this age than sleep through.
Babies go through regular growth spurts where they feed more often and wake more at night. Again this is completely normal. If you are bf then this is what keeps your supply up to date with what the baby needs.
Milk has all the nutrients and calories your baby needs. It doesn't matter how big he is. Milk, be it breast or formula is very high in calories. There is nothing, absolutely nothing that you could get down your baby at this age that would even begin to compare with the calories.
Waking every 3 hours is totally normal. I don't get why people expect babies to sleep through so early. It can take some of them 9 months or longer to work out the difference between day and night for a start. Babies aren't meant to sleep through early, their small tummies need filling regularly with milk.
I would leave it until baby can sit unaided and put food in their own mouth. These are signs to look for, not how often baby wakes.
Milk is by far the best food at this age most calorific and contains the necessary fortifications, you dont want to take space away from milk intheir tummies. Guidelines are due to increasing body of knowledge about infant nutrition so yes they change. 4-6 months is earliest but 6 months is better then you can give most foods without worry. Weaning doesnt make babies sleep through, my 8.5 month old is still waking for milk feeds but he is healthy and growing and thriving so it must be the right thing to do. Is your baby growing and thriving but needing lots of milk to do so? Thats whats important. Everyone I know waited until 6 months btw.