As I read through 3 John I kept coming back to the same question, why bother. It has all of 14 verses, and seems like a letter between friends that starts to say something, but then it doesn't. Why was is thought of as important to include this little note with all the other portions of the Scriptures?
To be honest I am not real sure, but I do know it is there and a part of what is offered to us. So what does it say? It celebrates the faithfulness of the people of God, and it warns against following this guy Diotrephes. Apparently Diotrephes was leading people astray and not playing nice. The most significant issue is found in the fact that Diotrephes would not have fellowship with the believes. We don't know whether he was believer of not, we do know he was acting like one.
The main theme of the letter is to say, I know things are rough, hang in there. When I come to visit I will address all the issues straight on. So what are we to gain? One thing, for centuries there has been a struggle for the followers of Christ to get along. Second, encouragement goes a long way.