The book of 3 John is the third of three letters that are included in the New Testament of the Bible that were written by John, son of Zebedee. John also authored the gospel that bears his name, and the book of Revelation. John was very close to Jesus during His ministry on earth and is referred to as, "the disciple whom Jesus loved".
This letter was written around the same time as 1 and 2 John somewhere between a.d. 70 - 100. In this short letter John thanks a church leader named Gaius for his support and contrasts acts of good with those of evil.