Honestly, I would not be bringing a new puppy into your home just yet. You have two puppies already. The Pitt female is still very young, and has much more training and growing up to do.
It's best to have 2 or 3 years between dogs. Also, Rottweilers are a breed that need much attention and training while young (first couple of years) and it's almost impossible to do a good job trying to train 3 puppies at once. You also will have to keep them separated from one another...so the Pitt and Rottie don't bond to each other. You want each puppy to bond to you.
Getting another female is a very bad idea. Pitts can be dog aggressive, as well as same sex aggressive.
You would have two females that are very close in age...it could mean once they are mature that they will fight...and maybe even try to kill one another. Could you live the rest of their lives having to keep them separated?
I really have to question any breeder that would be selling you a puppy in these circumstances.
I can't think of any COE that would want a puppy going to a home with two puppies already there,and to allow a same sex combination and breeds the way you plan.
Please re-think your choice of getting a Rottweiler puppy for now. If in the next couple of years you plan on getting another dog...make sure it's a male. Though the cockapoo (male) and Rottie may also have problems.
Gina