HELP - I am a little desparate! Blending 2 sets of ducklings PLEASE HELP

[zooomama]

I am so totally new! I have 3 (3 week old) male ducklings due to an error at the farm I got them from. They sent me 3 females to fix the mistake (now 1 week old). I now have 6 ducklings. The 3 big boys seem to want to bite the 3 tiny girls on the head and/or run away from them and then try to bite them on the neck. They act like they are afraid of them. So I have kept them separate and slowly tried to introduce them to each other. I seem to be making no progress.
ANY SUGGESTIONS for how to mix the two groups together?