I acknowledge OMV 3 is Beta, I have installed for testing purposes and not for anything critical. Please do not think I am complaining. I am reporting my experience and work-around I employed.
Installed OMV Version 3.0.26 with no problem and updated from Update management with no problem.
Installed OMVExtras latest version ... turned out to be V 3.3.4 ... Wanted to enable Mono Repos so I could experiment with Sonarr but the Switch to enable Repos did not work with a single left click like it did in Stone Burner. Had not enabled OMV-Extras.org Testing yet but doing that had no effect. It would have only given additional plug-ins versions that are still in testing.
Uninstalled OMVExtras ... Ran Apt Clean got fresh download of the installer and ran it again with the same results and still at V 3.3.4 and no left click to enable Repos.
Ran Update from Repos page and Version changed to 3.3.5 but still not having switch to enable repos working as expected.
Tried Double Clicking on the switch with the same result as highlighting the repo I wanted and selecting edit ... from that screen the option to enable is present.
After doing this Sonarr is now in the list of plug-ins.
I can live with this work-around to enable repos and it may be helpful if someone else has the same problem.
A solution to fix the problem correctly would be appreciated if there is one ... or it may work itself out with time and revision changes. This is really a Non-Issue since I now how to enable via the work-around .... after all you enable what you want and move on. Not going to toggle switches on a regular basis.
If the procedure involves uninstalling OMVExtras ... okay. Please be verbose in explaining what to do or give a link to a place I can learn more. I have not had any luck finding how to "Clear the Cache" or to "Uninstall and Purge and then Reinstall" - which may be the correct solution.