Not only can they get picked on ruthlessly but can be killed or made sick from stress. You should separate your different ages by using temporary fencing the plastic poultry fencing from Lowes works great Let them all live side by side for as long as needed might be 2 weeks could be a month or more but this way they see each other daily and can get use to each other presence. Once the 3 weeks olds are larger start letting them have time together for treats when you can be there to supervise , as you watch you'll know when it is safe for all to be together and you can take down temp fencing.
You haven't given treats? what are you inhumane lol treats well mine love dried meal worms and they are very special treats. they work well for when your working on getting new members into the flock because once you put them out on the ground they have idea anyone else is there they are so busy scarfing up the worms. Little English peas thawed ,
I hope you know I was just kidding you.
Most ducks love the little green English peas you buy frozen, I usually let mine sit out in the frig for a day then feed them when thawed you can also thaw them in the microwave. toss some in their pool mine go nuts for them even my geese and chickens love them.
