Like 2 John, this letter was directed to a specific person, Gaius. It appears that this name was common in the first century, so pinpointing the man is impossible. There are several men even in the New Testament who had this name, and we cannot be sure if one of them was the intended recipient.
John referred to Gaius with great affection in 3 John 1-2. It seems from 3 John 3-4 that he was either a new believer or possibly someone who had returned to living faithfully. In this short letter, John wanted to point out that godly living extends to how believers treat one another, “even though they are strangers” (3 John 5). It seems that Gaius needed a little prodding to support some traveling missionaries John had sent his way.
The second issue John needed to deal with was concerning a man named Diotrephes. He was apparently a narcissistic, arrogant man in the church who was bold enough to slander John and would not welcome the traveling missionaries. He even went so far as to unilaterally excommunicate those who tried to help them! This may be the reason John had to encourage Gaius to do this.
The statement about Demetrius, that he “has been testified to by all” (3 John 12), may have been John’s way to refute some of Diotrephes’ slander. This is similar to when Paul put his reputation on the line to Philemon for Onesimus (Philemon 17). This letter ends in a similar way as 2 John, with a reference to “pen and ink” and the desire to “speak face to face” with Gaius (3 John 14).