The reason I mentioned trying 1 at a time with your new ducks is breaking up the pecking order. You can leave one duck in with the chickens at night and put the new girl and one other in with her. After a few days if that goes well then do a switch so your other duck is in with the new one then after a few days put all 3 ducks into the pen they aren't usually in so it's not home turf so to speak. This will also give the chickens time to get use to the newbie with out being mean to her.