FCP X only ever supported 2 computer monitors.
What you're talking about is using two monitors configured as one screen, and the 3rd is configured as your second monitor. This is done in the System Preferences, under Displays.
Be aware that FCP X and Motion are very GPU dependent. If you hook two monitors to one graphics card, that card's resources (GPU power, VRAM) get cut in half and shared between the to independently. Applications will only be able to access the GPU and VRAM resources assigned to the primary monitor.
Hooking three cuts your graphic card's resources into thrids, so FCP X and Motion will lose even more performance.
What size monitors are you using, and what graphics card?