In my experience, the difference of adding a third GPU has been nothing short amazing (owned 5770 tf and 6870tf)
I have never encountered any issues with drivers(that I wouldn't have gotten with two cards)
Only one game out of my games list didnt scale to the third GPU(skyrim)
Microstutter became a thing of the past..(big issue with 5XXX and 6XXX cards)
scaling was very decent for the 6870's... not so much for the 5770's (but hey it was a present
) I would imagine its only better again with the 7XXX
(very good on intensive games such as metro, BF, crysis 2 hi res with dx11 and the witcher 2)
heat was the only real issue I ever encountered that was annoying... make sure your cases airflow is good and
maybe invest in a couple fans for your side panel