The author is "the elder" which is almost certainly the Apostle John writing near the end of the first century AD (for fuller explanation, see 1 John Author).
I. Salutation (1-2)
II. Faithfulness of Gaius Commended (3-4)
III. Hospitality of Gaius Commended (5-8)
IV. Selfishness of Diotrephes Rebuked (9-11)
V. Character of Demetrius Commended (v. 12)
VI. Closing (13-15)
F. F. Bruce, The Gospel & Epistles of John (Grand Rapids: Eerdmans, 1994).
Gregory Goswell, The Johannine Corpus and the Unity of the New Testament Canon, JETS 61.4 (2018), 717-33.
Jeremy Walker, 3 John with Jeremy Walker.