I'm guessing they would fair very well by how a pair of 780's perform but look up reviews and numbers for whatever games it is your thinking of running. I'm guessing some titles just are that demanding and others aren't that well optimized for doing it. I'm not sure any game has gone past using up 3gb vram yet? I remember watching loads of videos of 5760x1080 crysis 3 on SLI Titans, and they got to around 2.2-2.8gb I think, nothing that really warranted 6gb. You would have to spend a LOT more money to get anything superior to two 780Ti's, I would hope my 780's would handle it if called upon.
It doesn't really matter how you plug triple monitors in now the nvidia utility lets you sort it all out, it might change where your bios screen appears. I used to put the main screen and the left screen on the bottom GPU and the right screen on the top one so if any application didn't utilize surround/sli properly it would be slightly cooler running on the bottom card. If you want a 4th 'accessory' display I think that has to go on the slave card not in a DVI.
There's a good bunch of numbers on here -
http://uk.hardware.info/reviews/468...-5760x1080-and-frametimes!-5760x1080---medium
You can see 2x780's have an edge over 2xTitans on medium but that's closed up on very high. Even 3x 780's are still ahead of 3xTitans with only 3GB to play with so I think 780Ti's would be fine, but you can see how some stuff still isn't going to hold 60fps if you truly want ultra settings.
2x 780Ti's vs 3x 780's, that's the big question considering they are very close in price.