I'm only aware of having dual screen setups with the ability for a single accelerated window to span them all. When you go from 2 to 3 you need to start using seperate X screens with Xinerama (instead of nvidia's Twinview or MergedFB) to get the ability to span windows. Of course, this kills off any and all ability to run LFS in them, since OpenGL acceleration gets totally hosed. So, you're pretty much limited to dual screen, which if you're using TwinView/MergedFB it'll create one large virtual desktop that the LFS window can span across.