When you go over two gpu's you tend to add more latency to the system, as every card is working on rendering a frame that hasn't been displayed yet. So with 3 cards you're rendering at least two frames ahead of the input. With two gpus you're still rendering at least one frame ahead, but you only have to wait for a frame to be rendered before it's displayed half as often, so you end up waiting less.
If you are a competitive gamer, you want one very fast gpu in order to get the lowest possible latency. The GTX 780 ti is the fastest one you can buy right now so it wins. (If you wait for the 800 series, why not wait until the 900 series? any gpu is faster than one that hasn't been released yet.)