3 Monitor Setup - Display Order Priority?

 to lichinho: your trick works. But there is another solution. 
You have three monitors.
One is for a/v output. 
Open FCPX with a one window-layout.
Leave the main FCPX window where it is, or drag it to the monitor of your choice (but not to the dedicated a/v output monitor).
Now tell FCPX to show a/v output on your a/v output monitor.
Then choose - workspace - two monitors
The second monitor will now show the second window (overview, viewer, timeline) on the last of your three monitors.
Should work also with a vertikal oriented monitor. 
This is a long time bug in FCPX and should have been solved years ago.(Do you read this, apple?) I can hear the deep sighs of thousand of new FPCX editors, when they run in the "how to setup three monitors" struggle.

Thank you schloerg that really works.

A couple of years after my 2016 post up here, I started using this config. And, on my fourth monitor, I had Photoshop, sometimes Motion, etc. plus all required utilities specific to my ongoing project. It was nice.

The Mac was a recent loaded Trashcan and the software, always up to date. Please none that when I’d upgrade the OS, I’d have to horse around on the Mac with my 4 mini-display port monitor connectors. Sometime I’d need a couple of hours before succeeding (aka the 4 monitors working at their right place), sometimes it would need only a few minutes.

Then I replaced the Late 2013 Mac Pro with a Mac Studio, a computer advertised as happy with 5 monitors.

Unfortunately, I’m now back to 3 monitors. I need to confess that my 4 monitors are 1080p 32-inch HDMI displays from Dell and LG. I’m currently working on the 10th season of a WebTV project which doesn’t need 4k, yet.

After a couple of hours with empathic Apple tech support agents, I got to understand that HDMI could be the issue. The Mac Studio hates it. I understand, now, I’d need modern connectors, I.e. TB 3 or 4, USB-C … most of those technologies specific to expensive 32-inch 4k displays our producer has rightfully decided not to buy … for now.

The problem, « 4 times 32-inch » has spoiled me to a point where I can no longer edit with FCP X on my MacBook Pro.

So, as I said, I’m back to 3 monitors (one of them randomly blinking) as I was in 2013 when I started on our project with a 2010 MacPro that had two graphic adaptors.

On the 3rd monitor, I’ve chosen to place all required tools and datas instead of A/V Output.

Am I happy ? Honestly no. I’m the one who strongly recommended buying a Mac Studio, the Trashcan had become really despicable.

Am I unhappy ? Not really, the Mac Studio has some advantages.