Don't get too excited. This basically explains why it runs better on one card and not three.
"Without anti-aliasing, Samaritan’s lighting pass uses about 120MB of GPU memory. Enabling 4x MSAA consumes close to 500MB, or a third of what's available on the GTX 580. This increased memory pressure makes it more challenging to fit the demo’s highly detailed textures into the GPU’s available VRAM, and led to increased paging and GPU memory thrashing, which can sometimes decrease framerates.”
In other words it could simply be down to Kepler's larger vram.