if you add another 690, you'll have enough to run 3 monitors on 1080p for the very demanding games. 3 way SLI 780s will be good too, and if you want a comparaison, 3 780s on three monitors will perform close to 1 780 in 1 monitor in games that are well optimized. Battlefield 3 can be ran at 140fps in a single monitor with 1 780, I'm guessing around 80 fps in 1440p, so you'll be getting something like 60 to 70 fps in 1440p triple monitors on ultra, it will run games at 1440p on ultra settings, but in a couple years you will need to upgrade. Cheers!
ps: if there's some game like Crysis 3, don't go crazy on the settings, at 1080p you're fine, 1440p you'll have to play on high, maaaybe medium but high will do it.