I hope to start running LFS on 3 monitors, but ATM, it is on 2 17" (therefore 2560 x 1024 [not much more than 1080p) monitors. I get, I think about 90vFPS; I'd have to check though. But I believe this is a CPU bottleneck, more than anything. It seems that it isn't able to cope with the rest of my computer, albeit a Core 2 Duo based PC.
But IMHO, running 3 monitors, be it 1080p shouldn't be a problem with a 4890 or even a 5770. I've seen people doing this since the early dawn of multi - PCIe. If I remember correctly, I saw this person on Youtube 2 years ago [probably can still find] with 3 monitors (probably 19" 1440 x 900) on a 7900GT(X) and using a 6200 for the third monitor (the video displaying DFP and Act Labs shifter). And with almost twice the memory bandwidth and almost four times the processing power on a 5770, and four times the memory bandwidth on a 4890, 3x 1080p should be no problem!
EDIT: Back before native multi monitor support on LFS (Z28/25), I was running running 3840 x 1024 on a 7600GT (~70 FPS if I remember correctly (possibly more b/c I'm not sure if VSync was on to limit at 72).