I don't know why, but Devonthink seems to change its bundle / app identifier when upgrading. In general these should always stay the same, but maybe they want to support running multiple versions at the same time.
The easiest way to resolve this is to use the "Alle Trigger fĂĽr die App kopieren" (Copy All Triggers For App) option in your Screenshot, then use the "Eine der aktiven Apps hinzufĂĽgen" Option to add the new version of DevonThink. Afterwards right-click the new version and choose "EinfĂĽgen" to paste all your Devon Think triggers. If you don't need DEVONthink 3 support anymore you can then delete that from BTT.
I think I could add a "Move To New App Version" option to the context menu that allows choosing a new app or CAG (equivalent to the + button to add a new app)
Your CAG probably just checks for the app name "DEVONthink" and not "DEVONthink 3", but for exact matching without a CAG the app name is not enough - which is why that matching generally relies on the app bundle identifier (which for most apps stays the same for all time).