The Groups & Tags window in DT2 made it very easy to attach tags to files by dragging a file to the window and selecting the tag. I understand I can open tags in the sidebar to do something similar but it requires more steps. Groups and tags window was always open on my desktop and easily accessible. It would be great to have something similar in DT3
You can enter Tags in the Tools > Info > Generic inspector.
You can enter Tags in the Tags bar under the view/edit pane, shown via View > Tags. This is also quickly accessible via Control-Return.