I have seen the numerous 7950s being sold on eBay and am considering triple crossfire 7950 if it would be better than a gtx 780 assuming I can get three and the price of a 3 way crossfire motherboard for less than a 780. I am guessing a 780 is better but thought this might be worth a try.
Crossfire is notoriously glitchy. When you add to that the fact that those 7950s were probably run at 100% 24/7 doing mining, you're just not going to have the most stable experience.
3 way 7950's would probably beat a 780ti. The problem is though that 7950's are like 200$ a piece. And even then they are used very heavily for mining most likely. 600$ could buy you a brand new and highly binned 780 that you could overclock the piss out of.