As I specified above in the comments, if you would like your tri-monitor setup to only be full screen in the center and have your far left & far right monitor viewing the desktop (for multitasking purposes), do the following:
- Go into settings, Video: Make sure you are in Fullscreen Mode
- Recommended 1920 x 1080 resolution, but it is up to your hardware and personal preferences for display resolution
- Be sure to save those settings
Now, if this hasn't happened already, you should notice that your computer has grabbed focus of Diable III. The far left and far right monitors are black - with the center monitor showing your game.
- Now press Alt + Tab to get focus of your desktop ( this should minimize your game)
- Click the current running [ & hidden ] game of Diablo III to regain focus of that application
- This should now have your game in the center screen, with your desktop on your far right and far left monitor
If this does not work on your first try, try again until you gain focus of your game with your desktop on the other two monitors. Your system, by default, will want full focus of the game: hence why it blacks out the other two monitors. From my experience this occurs in many other games today with a multiple monitor setup and Alt + Tab is the easiest way of gaining viewable access to your desktop.
Note: It does not matter if you have your video card setup in eyefinity (or the NVidia equivalent of one giant desktop across 3+ monitors)