There's simply no way it'll be three times faster. We know Samaritan ran on 3 GTX 580's but we had no info about the fram rates, and how well it would run with just two of them.
Let's say it ran at 60 FPS with 3 GTX 580.
Then, even if Kepler is twice as fast as GTX 580, it can still run the demo at 30 FPS, which is reasonable.
And if you consider that FXAA is a lot lighter on the GPU, then the gap between Kepler and GTX 580 diminishes further.
And finally, for all we know, it could be a dual-GPU Kepler, like a GTX 690. Though that's unlikely.
But as much as I would love that to be true, there's no way it's 3 times faster than the GTX 580.