Exported an RTF file to a Template and put it in the Template Folder (~/Library/ Application Support/DEVONthink 3/ Templates.noindex/Toolbar.) Then dragged the Template into the Customize Toolbar window so it became an Icon, quit and restarted DT3, and then dragged the Icon onto the Toolbar. Deleted the Icon from the Toolbar but was unable to delete the Icon from the Customize Toolbar window by dragging it off of the window the way all other Toolbar Icons are deleted. User error or small bug?
You have to remove the template from the templates folder too.
Thanks, @cgrunenberg, that worked. There is a similar thing that I need to ask about but will do on a separate topic.