why you are thinking to get the third one?sli gtx 560ti is ok or if u want more then sell both gtx 560ti and get hd 7950 if u can.
I was looking in to getting the latest Radeon in the 7000 series, totally forgot I could sell my GTX 560 Ti's.
But! I want to give a friend one of my 560's. So I will be getting only around 200-250 back. That is a great Idea and one I can consider. Thanks.