Yes, you will be able to keep the new baby chicks in the same coop as the three-month old chicks.
However, you will need to keep the babies separated by a see-through barricade for their safety as the older ones are certain to thrash them. It's what big chicks do to baby chicks, and especially since there are so few of each group. It's even more likely that the large ones will focus their aggression on the babies.
It's advantageous to raise the new babies along side the older ones since it will give them all time to get to know ones another and by the time the babies are ready to mingle with the older ones, they will be considering themselves as a flock.
Your best chance for a safe integration after the new chicks get in the range of three or four weeks old, would be the panic room method, using smaller entrances from the baby pen into the rest of the coop or run. I like to raise my chicks in my covered, weather-protected run since it gives the flock more room and opportunity to be exposed to one another. By the time the chicks are successfully integrated, then moving them into the coop to sleep there is pretty easy. But if you aren't set up with a protected run, raising them in the coop is certainly doable.
For more details on brooding chicks in a run alongside older chickens, read my article on outdoor brooding linked below my post.
