I've got a flock of 10 week olds (14 of then) and I've got a flock of 3 week olds (18 of them).
Is it possible to mix the 2 flocks at this age or is 3 weeks old too young? Or is the age gap too big? And if I do mix the 2 what are some concerns I should look out for?
Is it possible? Yes, it is possible. It might work, it might not.
Concerns:
Sometimes the older will kill the younger. It could be just one of the older will actively search out a younger to kill or they could operate in a mob. A lot of this has to do with the personality of the older and you cannot tell that until you try.
Sometimes they mingle very well but often they form two separate flocks. If the younger get too close to the older they might get pecked, so they soon learn to keep their distance both during the day and at night. They need enough room to avoid the older. If they don't have enough room to run away if they get pecked (a warning to get away from me) or enough room to stay away when the older flock is wandering around then they can't take that warming and get away so the older can continue pecking.
A peck to warn them away isn't a big deal as long as the chick runs away. If you see a chick getting its head continually pecked that is a critical situation. Pecking a hole in the skull is how they kill them. If you see that behavior, stop it immediately.
The more room you have the better, both during the day and at night. Adding clutter can greatly improve the value of what room you have. Clutter means things they can get under, behind, or over to help break line-of-sight. You do not want them to get trapped so make sure they have a way to escape if they get chased.
Having separate feeding and watering stations can help so the young can eat and drink without being bullied by the older. This does not mean separated by only 1 meter. It means very widely separated, maybe in the coop and in the run. It really helps if you have clutter to break line of sight between the feeders and waterers. I have a feeder and a waterer in the coop, two more feeders and two waterers widely separated in the run.
I'd suggest housing the younger where the older can see them for about a week so they can get used to each other before you turn them loose. Hopefully you already have that. This gives you an immediate place to put them if it doesn't work out where they can still see each other. Then turn them loose to mingle one morning when you can be around to observe.
People do this successfully but sometimes chicks die. I've had broody hens wean their chicks at three weeks, leave them alone with the flock to make their own way. The flock included adults and teenagers. You don't get guarantees one way or the other with living animals but you have a decent chance at success. I'd try it.
Good luck!