A 6950 crossfire or 6970 crossfire is more powerful than the GTX590 or 6990.With those dual GPU cards they had to tone down the clock speeds so they could fit it within the power requirements.They really have more negatives than positives.And the price you pay for a dual GPU card can really be justified since a 6950 crossfire can do the same thing for half the cost.
I would suggest going with a 6950 crossfire since the extra VRAM will help with the larger resoultions.
Where are you getting the 2760x1080 resoultion from? A triple 1080p setup is 5760x1080.
Three monitors is really great.If you have the money I really suggest going for it.Yes at such a large resoultion it will tax the GPU a lot but if you get a 6970 crossfire it should handle it well.As an alternative you can always get a 720p 3 monitor setup.
x3 20" monitors should give you a lot more FPS but still have the same affect as Eyefinity.