A baby this young, does not have a fully developed digestive system... adding cereal to the bottle will not help. It is a myth.
At this young an age, they wake. Even 1 year old wakes.
3 months is a growth-spurt time, and they need to feed to keep up with their growth. And, sometimes babies "cluster feed" meaning they need to feed even every hour.
I know its hard... but keeping him awake, when he needs to nap, is not going to help. Because, an OVER-tired baby/child, actually does NOT sleep well and they wake more and they do not have a good quality of sleep. You need to go by your baby's cues... when he is tired, then he should nap. A baby this age, typically gets tired about 2 hours after being awake. So you need to gauge his sleepy cues... then put him down for a nap.
Or you can try baby sleep sacks. Some babies like that. Its like swaddling.
But, if a baby wakes and is hungry, you need to feed. And it is not easy, and tiring. But regularity helps. Have a daily routine... with your kids. Is your 18 month old napping? She ideally should be. And try to put them BOTH down to nap at the SAME time. It is possible. I did that with both my kids. It just takes getting a routine about it, and knowing when your babies are tired. Not waiting until they are over-tired.
And if you can, get a friend/relative to help you.
Are you a single Mom or have a partner? If so, he should be helping too... a Mommy needs rest.
All the best,
Susan