If you have outputs for 2 monitors, but only one monitor is connected, then set one monitor tab, (in MA3 Display Settings), to:- 'Display nothing on this monitor'. -
Wouldn't it be better to fix the underlying problem?
If only one monitor is connected, but the system thinks that 2 or 3 are connected, then something is fundamentally "not right".
[My video card has 2 monitor outputs, and is capable of driving 2 monitors at the same time, but only 1 monitor is connected. The system knows that, and MA3 doesn't have "monitor tabs" in display settings]
Clearly, telling MA3 to not display on the unconnected monitor should make MA3 work better, of course.