Are all the three month olds males? I would not risk putting any Three month old females in with older Drakes - lock up the older drakes or something until the young ones are 4.5 months… I have already got my adult Drake separated from my flock, but gets three or four of his favorite females in his pen with him each day ( just not any of the elderly ones that he harasses and definitely not the young ones that aren’t 4.5 months yet )It is now June, in July we're merging the adult Khakis and our current generation of Welshies and Appleyards when they turn 3 months old.
We're a bit worried about the small Welshie that's about 2/3 size of a regular one ... One of our Khaki drakes is really a bully.
However -- I've noticed that in the mixed group of Welshies and Appleyards, the small guy tends to walk around with 3 Appleyard bodyguards) (which are twice his size in length and 3-4 x in total body volume) So it might all turn out well.