Or it could be that you did an upgrade in the meantime. It’s a bit difficult to explain.
You say you have a Dorico 2 installation where Play works. If you had an older Play, then D2 would have checked that and may let it through, ie not blacklisted it.
In that case D2 won’t check it anymore, because it is in the module cache. So even after an update to a newer Play, D2 will not rescan it.
Now that you have D3, there is no module cache, yet, and so it scans Play. Now the scanner might detect it as “bad” and blacklists it.
Could you try with Dorico 2? First go to ~/Library/Preferences/VSTAudioEngine2 and delete the file ‘Module Cache.xml’,
then start Dorico 2 and check if Play turns up. The deleted cache enforces a rescan.
In any case, please create in both, Dorico 2 and 3 a diagnostic report (do Help > Create Diagnostic Report which creates a zip file on your desktop) and attach here in the forum.