actually, you CAN. The problem is, the third card won't be that dramatic of a difference. Knowing him though, if it can be done, he will want it done. I'm almost certain you can do it. If it doesn't work, i will drop back to 2 but right now its looking like 3.
I was certain that you can only have 4 GPU max when during SLI or Crossfire.
I heard that 680's are better then 690's. here is a link - http://www.videocard...h_end_gpus.html
Just my 2 cent
there is no way a 680 is better then a 690. A 690 consists of 2 680's......i don't know who's results those are but they won't be mine. I personally own 2 GTX 580's and its showing the 690 BARELY better then that one. I think not LOL.
It depends on the situation as well. What games you are playing & resolution. You should be fine with 2x 690s 4gb.