RekordBox Video output monitor.

We are not sure we understand your configuration.
Does the computer have internal monitor and 3 external monitors?
Do these monitors show as individual Desktop?
 
We expect the usage with one external monitor and don't expect the environment with multiple external monitors.
But we suppose rekordbox shows the video output window on the same video output (not same monitor) as before.
If the video output window of rekordbox appears on random external monitor, we suppose the computer detects the external monitors with different order every time the external monitors are connected.
 

I have a macbook pro where I use the main screen and two external monitors with individual desktop.

RekordBox randomly uses monitors.
I was able to use the video output on the monitor I needed just by clicking on the video preview window in RekordBox and in this case a small output window is displayed where is possible to drag it to the desired monitor.