I have been in your exact situation before, but with 470's. I had to spend 200 for the 3rd 470 and another 230 or so for the 1200W psu upgrade. So at least 500 by the time i was done after shipping. I should have just sold the 2 470's and picked up a 580 at the time for just 150 more to make up the difference.
Sooo....Spend ~3-400 on 3rd gpu (and psu) or sell 6950's and buy 7970 for maybe another 200 after
