I've never used multiple monitors for gaming so I can't check, but there's a setting in conf.ini that might help. It's in this section of the file:
Try changing the "Use3Renders" setting from 0 to 1 and see if that helps.
Bear in mind that you may have to start a mission to check, the wide and multi monitor support may not engage in the menu screens.
If that doesn't work, check that your Windows desktop setting is set to span the desktop across all three monitors, the setting that causes applications to span all three when maximised.
If it's still not working as desired after than, I'm stumped.