Final Cut Pro - 3 Screens - Choose which monitor is used

We have three displays for a church I do contract work for. I've found all must be connected to the Mac and powered up before booting the Mac. When we do that, the configuration we set up in System Settings sticks. If a display is not connected and recognized at boot time, it can throw off what's assigned to what.

AND this is not an FCP issue, it's a macOS issue, so you may want to post too that forum, also.

This question comes up from time to time.

(Note to self: I probably should write a user tip about it :-))

Here is how you guarantee that the screen you want is used as "secondary display" in Final Cut Pro.

1) Go to System Settings->Displays, and drag the little menu bar representation to the one you want as secondary display in FCP.

2) Start FCP in the display that you want to use as the primary display for FCP. (if it starts on a different display, just drag the FCP window to the one you want)

3) Window->Show in Secondary Display->Viewers (or Browser, or Timeline)

4) er... there is no step 4

I would not hold my breath on this thing. In 14 years, Apple has never offered the possibility to select which display to use...

There may be a "gymnastics" that may achieve your desiderata, but I don't recall the exact order, and I don't have three displays at the moment in order to test.

I vaguely remember that choosing a primary display, then another in some sequence would make it work, i.e. somehow make a display be the second.

Another possibility that might work (again, I cannot test) if your third display is a 1080 or 4K, to nominate it as A/V Output. With A/V Output turned on, the display will become unavailable for use as "secondary", as so the other one has to be used. Hopefully then, turning A/V Output off should not revert the secondary display status.