I don't have enough "reputation points" to just add a comment to the above answers, so I guess my only options is to post an answer even though it is really just a way to speed up the process above.
Instead of fully rebooting, you can just restart explorer. I created a batch file to do it, then made a shortcut, then assigned a shortcut key. This batch file and shortcut will either have to be on your desktop or in your C:\Users\userid\AppData\Roaming\Microsoft\Windows\Start Menu for the shortcut key to work.
Screenshot of restart.explorer.bat Batch file, Shortcut and Shortcut Properties
I also made a point of having my laptop closed and only my one monitor plugged into my dock when I did this to make sure it was set as Monitor 1 as that is what I wanted. It kept this number even when I opened my laptop and added that screen, at least for me.
You can also have regedit open and monitor the Windows Registry keys above and use F5 to refresh and see each monitor as it is added.
Then you can just delete the new entry in the registry and try it again as you experiment.
CONFIGURATION key will load a new entry for every combination of monitors you create. 1, 1+2, 1+2+3, 1+3, 2+3, etc.
I'm not sure how Connectivity Key works, but likely something to do with type connection.
MonitorDataStore and ScaleFactors will have 1 entry for each unique monitor you have ever connected.
All 4 of these keys can be "blown away" and they will rebuild as you attach monitors and change configurations to extend, duplicate, etc. across multiple monitors.
