As for running 3 monitors over 2 video cards you shouldnt have any problems at all, and as for the SLI thing you can turn it off and on in the nvidia control panel and I've read that some users set a macro key for turning it on and off (not in wow of course but on their fancy keyboards)

Now as for the 3 monitors I would recommend putting your 30" on a video card by itself as the resolution you would be running would suck more juice out of that video card and if it ran that and another monitor you might have performance issues, so run the two smaller monitors on the other card if at all possible, and the software end should do everything else for you.