You have come to the right place. There is a great duck forum, populated by some expert duck folk, on this site.
Three male ducks MAY get combative with each other when adolescent hormones kick in, but I have runners and have rarely had any problems with boys who grew up with each other. Also, my runners free range with my chickens, and maybe because they have plenty of female ducks to "romance," have never sexually assaulted my hens -- although I do know that can happen. They don't sleep in the same house/shelter.
I would not put that little chicken in with the ducks. She's probably already traumatized enough.
Drake feathers are a good indicator that yours are boys, as is the difference in their voices, as previously pointed out.
Welcome to BYC, where I hope all your questions get answered!