The 560 Ti was nVidia's top selling card last time around.....it's market sahre among gamers was bigger than all DX11 AMD cards in the 68XX and 69xx series put together. Last I looked at the cards hitting Steam Servers as of end of February .... The 560 held 10.25% market share among DX11 cards and the 4 AMD cards totaled just 10.15%.
As well as it was accepted in the market place though, I still think it's a bit weak for 3 screens....even with two in SLI if you're playing the more demanding games. To cross 3 screens with a single card, I think I'd wait a bit and see how the 660 Ti stacks up against the 78xx series.....but that will still be above $200.
A single screen is 1/3 the amount of pixels as a singe screen, so 3 times as many "dots" to move around...that's quite a load increase.