Although 2 John & 3 John are extremely brief, both are clearly structured with an epistolary opening, body, and conclusion, as found in most of the other "letters" of the New Testament and in the ancient world in general.
2 John
Letter Opening (vv. 1-3)
Sender & Recipient: "Elder" to "Elect Lady" (1-2)
Greeting: grace, mercy, and peace (3)
Letter Body (vv. 4-11)
The Love Commandment (4-6)
Deceivers vs. True Teachings
(7-11)
Letter Conclusion (vv. 12-13)
More to write; plans to visit personally (12)
Final greetings (13)
3 John
Letter Opening (vv. 1-4)
Sender & Recipient: "Elder" to "Gaius" (1)
Greeting: joy in the truth (2-4)
Letter Body (vv. 5-12)
Hospitality toward Christian Missionaries
(5-8)
The
Bad Example of Diotrephes (9-11)
A Commendation of Demetrius (12)
Letter Conclusion (vv. 13-15)
More to write; plans to visit personally (13-14)
Final greetings (15)
[Note: Since 2 John and 3 John have only one chapter each, proper
biblical references for these letters list only the verse numbers, without a chapter "1".]
Although 1 John is usually called the "First Letter of John," it does not follow the standard structure of an ancient letter (it has no epistolary opening or closing). Rather, it is more like a "homily" or "sermon."
1 John (R. Brown, 1988)
Prologue (1:1-4) - cf. John 1:1-18
Part One: Walk in the Light of God (1:5—3:10)
Walking in light (1:5-7)
Opposition to sin (1:8—2:2)
Keeping the commandments (2:3-11)
Opposition to the world (2:12-17)
Antichrists (2:18-27)
Children of God (2:28—3:3)
Avoiding sin (3:4-10)
Part Two: Walk as Children of the God of Love (3:11—5:12)
Keep the commandments (3:11-24)
Test the spirits (4:1-6)
Love (4:7—5:5)
Witnesses to the faith (5:6-12)
Conclusion (5:13-21) - cf. John 20:30-31
1 John
(D. Moody Smith, 1996)
Prologue: The grounds of the testimony (1:1-4)
The true message of Jesus (1:5—3:24)
Fellowship, obedience, and forgiveness (1:5—2:17)
Warnings against false teachings (2:18-28)
The marks of life in the community (3:1-24)
Testing the claims of those who testify (4:1—5:12)
Testing the spirits (4:1-6)
Love as the essential test (4:7-21)
Obedience to the commandments (5:1-5)
The true testimony (5:6-12)
Postscript: Sins, forgiveness, and certain knowledge (5:13-21)
Brown, Raymond E. An Introduction to the New Testament. New York:
Doubleday, 1997. - Ch. 12: "First Epistle (Letter) of John" (pp. 383-94); Ch. 13: "Second Letter of John" (pp. 395-400); Ch. 14: "Third Letter of John" (pp. 400-405).
Painter, John. 1, 2, and 3 John. Sacra Pagina, 18. Collegeville, MN: The Liturgical Press, 2002.