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.