I do not own Crysis 3. However, Crysis 2 is also running on Cryengine 3, if I remember correct and I do not have any problems running Crysis 2 at 5400x1920 (25% higher reso than 4K) with just a single 7950. Have not looked at frame rates, but it feels smooth.
I did downgrade resolution a bit later to 4500x1600 though as after switching into windowed mode was irritated by my task bar covering the edge of my window (this is approx 90% of the 4K reso at 7.2 megapixels vs approx 8 in 4K).
Settings were done manually with motion blur tuned down (as I dont like that effect) and particles and something more turned a notch down if I remember correct.
But as I do not own Lighting nor Crysis 3 then I do not know what they are doing in there and cant really comment on it other than pointing out that in Crysis 2 there is no problem running it at 4K with just a single weaker card than this newest Lighting. Now I can't comment also on how things would play actually with triple 4K - it would be sort of like testing something with 3 screens at 1024x768 and then going ahead and talking about how it would perform at 3x 2560x1600. But Vega or somebody will have triple 4K sooner or later I believe
Just not on these Ligtings, unfortunately, because of crippled output connectors.
About games performance, in general, at 4K resolutions - in my experience it's not as bad as people make it sound. Even a single 7870 is enough to play at 4K even in pretty recent games (Metro 2033, Crysis 1 and 2, etc) if one does the settings manually. Sure, if you dial straight for preset "ultra" some of the effects applied in presets kills the performance, in my experience it's usually the unneeded x16 AA applied by preset at highest settings. Even FXAA should be capable of killing performance when applied at x16 level on 4K regardless of card(s) used.