This is going to be an issue with nUI5 as it tries to fit itself to the full width of the display and with your change, the display is three monitors wide which is making nUI try to be three monitors wide.
That said, there are other users using similar setups that have nUI working. It takes some tweaking. See this post for instructions on how to do it...
http://www.wowinterface.com/forums/s...ad.php?t=20917
nUI6 has built in support for viewports to address this problem, but modifying that is not currently possible in the alpha.