Final Cut Pro allows you to use the application across two connected displays. While Mac OS X supports numerous displays, Final Cut Pro cannot display its windows on more than two computer displays at a time. This includes Digital Cinema Desktop mode.
For example, if you're using two displays for the interface windows, you will not be able to use a third display for Digital Cinema Desktop. You can set up a better, similar configuration by using your two displays for Final Cut Pro interface windows, but use a broadcast monitor for your main video display.
For more information on configuring Final Cut Pro with your hardware, refer to the Final Cut Pro User Manual, available from the Help menu in Final Cut Pro.