I only have my two boys, so I can't help you with that dilemma (but think a lot about getting a little girl). 6 months is a great time to introduce a new puppy. Bentley was 6 months when we got Griffin and it was a very easy transition as they are both at playful ages and are puppies, but are independent as well. My vet told me that as long as a dog is under two years old, the transition should be fine.
A good argument for getting a third is that you can take one with you when you go shopping, etc. and there are still two at home to play while you are gone. I have this problem right now because they both want to go with me everywhere and i don't want to leave one behind so I take them both with me, but it is hard to maneuver with two dogs and I can't find the right bag to carry them in (it also gets pretty heavy). Having a third would solve this problem
Good luck - I'm sure you will make the right decision.