Just designate the middle monitor as your "Primary" under whichever graphics settings you are using, then unlock your taskbar and move it to another monitor as you would if you where re-positioning it on the same screen, lock it again and voila - gal civ plays on the middle screen and you still have access to your taskbar, start menu and programs on the other two screens.
TL;DR - it's what I did on my setup, except with just two monitors.
