Make the Inbox(es) of the destination database(s) into Favorites, then drag the document to a favorite in the sidebar.
Same as (1) but instead of drag, use Move To… in the contextual menu and choose a favorite – helpful when the sidebar is hidden.
Use the Data > Move To… command (or ^⌘M) and use the small popup that appears there to search and place the document.
Drag the document out of DEVONthink 3 and onto the app’s icon in the Dock, so that the group.
I always suggest: read the manual, read Help, read Kissell’s book, or just click around and experiment. 99% of questions asked here can be answered quickly by experimentation. That’s how we all learned – it’s interesting and well worth the investment.