We just added a third doggy to our house two days ago! It's been a total zoo here and I'm exhausted but I just know that this new puppy will be a great addition. My female adult growls and barks at him and so we watch her pretty closely just in case. The puppy just wants to play with everyone and explore everywhere.
We introduced him very early - the breeder allowed us to take our dogs with us to meet her and the new pups at 4 weeks (our dogs didn't meet them then, just went along for the ride) and then at 6 weeks they actually met in person ---
They didn't know what to think.
And when we brought him home, we put him in one of those wire pens that you can get at the pet store and let the big dogs go up to him and sniff. Kodiak (our red/white) wanted nothing to do with him and skulked off to pout! Quinn growled but was curious.
This morning was the first time Kodiak tried to play or give any attention to the puppy...and it's only been 2 days so that's a fairly good sign.
I think just monitoring them (which I've been doing nonstop - the only reason I'm online is because all 3 are sleeping!) carefully and making sure that your other two dogs get LOTS of attention. The puppy is all cute and needs attention, but he needs to know his place in the pack. The first two are above them in the pack and it has to realize that. And your first two dogs will get used to him and establish dominance.
Let us know what you decide! =D