there is another way to have a newone for Pi3?
There isn't any package for "plexmediaserver" on network
You also could add the repo and key to the repo tab in omv-extras to eliminate any command line steps. And not to nitpick, but you really shouldn't add repos to sources.list. That is what the sources.list.d directory is for.
can you please give an example with parameters on OMV-EXTRAS? thank you