It all depends on how the dogs are introduced, as well as the breeds. As for getting a female, it might add some minor issues like dominance, females tend to be more dominant. You would want to look for a dog that's more submissive than your current pack, as they have already set in their ways. But other than that, a 3 dog pack isn't bad at all. I don't think even numbers or odd numbers is even an issue. My trainer has 3 dogs herself, just added a golden to the pack. Her main 2 were a border collie and a sheltie, now she's also got a golden retriever. The golden is a male, and the other 2 are females. Just make sure you introduce all dogs, and make sure they get along.