Hmm, since this is the closest thing to the SoftTH thread: I've managed to get triple screen working in 3D even without my upgrading my board. What confuses me is the manner in which I got it working.
I have tried the tip to use SoftTH with a dual-screen setup, by commenting out the appropriate monitor device ID, thus letting me have the FOV centre back in the middle of the primary screen (instead of being where the two monitors meet) - this has worked great. Inspired by this, I finally ordererd my new board with a second 16x PCI Express slot. With that due to arrive tomorrow I thought I should test out the second graphics capd and third monitor, so I know they're still working.
I gave SofTH another shot on the off chance and, as before, LFS won't output to the screen and the SoftTH logos flicker indefinately. So I went to comment out the right monitor again, so that I could at least still use two displays with three attached and by some miracle, all worked as before but with 3D output on both the left AND RIGHT monitors. In short, it works! Odd, huh?
There are only two issues as it stands:
1. I have to manually reduce the resolution of the right monitor in windows beforehand, else LFS appears in a small box on top of my desktop background.
2. My frame-rate has halved with the addition of the third screen.
I'm hoping my new board will cure at least the second problem, else I have ordered a new mobo for no reason (apart from supporting SLI it's otherwise identical to my current board).
Perhaps this post will be useful to Kegetys, or others who have suffered the same problem as me.