Synthetic benchmarks put 3x780 above 2x780ti but in most games 2x780ti and 3x780 will give you roughly the same framerates. I'd be inclined to just go with another 780 and save yourself the $1000+.
But if you are rolling in it and can't spend your money fast enough then there are advantages of picking the 2x780ti: less power draw, better heat management (and thus maybe more OC ability), and the option to get a third 780ti later on if you want to.