You didn't list your system components (specs + PSU) so I can't recommend either until I know what your system can handle.

Your system should be able to handle three way SLI but a two way 780Ti should be more than sufficient to run any game currently available especially on a single monitor running at HD or 2K.

To me, the sweet spot is 2 card SLI. Adding a 3rd or 4th card doesn't seem to scale as well (performance return for the $ invested starts to diminish), plus one typically has to invest in a 1200w or greater quality supply to feed everything properly. Cooling also becomes a bigger problem with 3 or more cards sandwiched in a case. You've got the power supply covered, but take a look at gaming benchmarks for a x3 780 setup vs a x2 780 Ti... Odds are they won't be that far from each other, especially with the higher core clocks most Ti's seem to be able to handle. Yes, 3x 780s will be a bit faster, but I highly doubt at 2k resolution you would need more than what 2 780Ti's can provide, which is all the performance necessary to drive any game out there at max settings with plenty of margin to spare.

Supercharged_Z06I highly doubt at 2k resolution you would need more than what 2 780Ti's can provide, which is all the performance necessary to drive any game out there at max settings with plenty of margin to spare.

That would depend on the game, new games on the horizon like Ubisoft's watch Dogs will tax multi gpu's no doubt. I tested Crysis 3 on my pc with i7-3820 at 4.6 GHz overclock and there are times when my 2 780's have no margins, maybe the 780 ti's could handle the game with margins?, i have seen titans in 2 way sli only managing mid 60's to 70's, it really depends on the game, future pc games like the one mentioned will be more taxing on current hardware. I only game at 1920x1080 and 120Hz refreshrate and the 780's can be pushed, although i could maybe overclock them in the future when games get tough going, of course i'm going to need much much better cooling though.