This is one of those rare personal letters that made it into the canon of Scripture. If I’m not mistaken, I think there are only 4 others, all written by Paul (1&2 Timothy, Titus, and Philemon).
As for this letter, the short hand moral lesson would read: Be like Gaius and Demetrius, not Diotrephes.
The last tidbit I would add is that John ends his 2nd and 3rd epistles with remarks about not wanting to write much more. Rather, he wants to see the people face to face. I think that’s important. Don’t let the medium of text get in the way of face-to-face relationships.