I've been told the same thing about female/female and male/male combinations - and it can be true, but there are also exceptions. Working with Arkansas Boxer Rescue, I got to foster Buddy - and he and my Bailey never did get along very well. There was always tension and there was one big fight. So, I wanted to try a girl - and the lady I work with at ABR said I could, but that she was concerned because I already have two girls. So, we agreed that I'd try it, and that we'd just be careful to be prepared for aggression issues. Well, you've never seen better buddies than Lexy, Bailey and Maggie. Lexy is totally neutral, but Bailey can be a bit bossy - so I knew that whoever I ended up with would have to get along with her - and Maggie is a perfect fit! They play and play and play and there is none of the tension that was there with Buddy - and Buddy and Maggie are about the same age.
So, sometimes, you just have girl-friendly girls. It can happen. Obviously - my vote it that you keep her. Sounds like she'll keep your wild dal busy and let the older two have some peace!