3rd John is the twenty-fifth book of the New Testament and the sixty-fourth book of Bible. It is a letter written to saints (especially Gaius) from John the Apostle  to encourage them to not imitate what is evil, but what is good .
The summary is that the book is written to Gaius, someone in a certain church. It is for the supporting of the church. Yet, a Diotrephes rejects what John wrote to (may be John 2), and also rejects a believer coming over.  John wills to see Gaius face to face as he wasn't going to write more.