The parts:
I. Preface, verses 1-2.
II. The discussion, that is, a commendation of the faith and beneficence of Gaius, a reprehension of Diotrephes, and praise of Demetrius, verses 3-12.
III. Epilogue, verses 13, 14.
Heidegger's Bible Handbook: 3 John: Synoptic Outline
A New Testament Introduction! www.fromreformationtoreformation.com/new-testament-survey