Richard Heard , An Introduction to the New Testament, , Chapter 22: The Johannine Epistles;
There is evidence these epistles of John were written by the author of the Fourth Gospel. The purpose of the first epistle was to build up the faith of a community known only to the author. The second epistle is a warning against those holding heretical views. The third epistle threatens to expose Diotrephes who refused hospitality to some missionaries.