I'm just guessing, I imagine fences is best if tied to the Icon spacing settings. That may not be the way it's done. I don't use Fences.....I prefer to hide my Desktop Iconsand use Tiles.
See if adjusting the spacing makes a difference (while in spanning mode)
Control Panel > Personalization > Window Color and Appearance > Advanced Appearance Settings > Item Dropdown
3 monitors, at resolution 6036x1200
Odd resolution.
Awesome!!!! Bet that's expensive!!
Less expensive than a crossfire setup...BUT you do need to purchase an active display adapter to make it work...
I'm sure a HD 6850 would work good and they run about $160. Add $30 for the adapter.