Personally, my third monitor on my main PC is solely there to monitor my rigs in small teamviewer windows with only local connections enabled.
I have an icon for each rig on my desktop to connect ("C:\Program Files (x86)\TeamViewer\TeamViewer.exe" -i 192.168.... --Password password) and have a macro that automatically resizes and moves all the windows neatly on the 3rd monitor.
I know there are some specialized rig monitoring tools but I prefer it this way.
I just set the onboard GPU as primary on the rigs so I can change resolutions and I don't need to plug a monitor into them. Though, I use nvidia cards and I have no idea how AMD would work.
For amd if you set onboard as primary you can't access the cards data like voltage , memory,fan , temp .....etc , you have to use dummy plug