Your numbers will help a lot. I always think of it as a number game of pluses and minuses
To begin with, you have 8 birds in an established flock in their home turf. so say + 16, but the are going to be much smaller that the new older hens so -8 = + 8
3 incoming hens, are bigger and are friends +6, but are coming into a strange place so minus 2-3 so say +3
Not too much difference, so it should work fairly well.
Couple of big points though, do you have enough space? Cause those 8 week birds, are still small, and will grow, needing more room! Often times what seems to be a personality problem, is in reality a space problem. Measure your set up, divide the area of the coop by 4, and the area of the run by 10 and those are a starting place for the number that will fit in your set up. Even if you plan to free range a lot, you need to have enough space to lock them up if you need to.
In the run, make sure that there are multiple hide outs, roosts, places where a tired bird can get away from the others, a pallet up on blocks, a roost stuck kitty corner through the wire, can all help make for more happier flocks.
Space is crucial, if you don't have enough, don't add more birds, it causes rather horrible wrecks.
Mrs K