I agree with Diane. Years ago we had 2 female GSDs introduced a male puppy GSD and a year or so later the middle female starting antagonizing the older female. I guess it was a change in the pack dynamics. It was nothing serious but the behavior never surfaced until the third dog was introduced. Of course I know people that have 5 or 6 dogs that all get along fabulously. I think pack leadership and training is key.
Kater's Mom (Sara)