This book is so called because it is the third of three epistles by John.
It is generally agreed that the writer of this epistle was the apostle John. He calls himself "the elder," and the closing verses are almost identical with those of the second letter. The points presented concerning the authorship of the second would also apply to the third epistle.
Third John is addressed to an individual by the name of Gaius (1), a personal friend of the writer. The identity of the person is hard to determine. Although some connect him with the Gaius of Romans 16:23 and I Cor 1:14, it is difficult to identify him with any other person thus named in the New Testament. He was probably a convert of John (3,4). Gaius was known for his Christian hospitality, having cared for "brethren and strangers" (5).
Third John was probably written from Ephesus some time after First John, perhaps toward the close of the first century.
There was some misunderstanding about receiving certain evangelists. The object of this letter was to commend to the hospitality of Gaius some Christians who were strangers in the place where he lived. These Christians probably carried this letter with them as an introduction to Gaius. The contents center around three men: Gaius, a charitable Christian; Diotrephes, an uncharitable church member; and Demetrius, another worthy Christian. The letter is of special importance because it affords us an insight to the church toward the close of the first century, giving both excellencies and defects, noble and ignoble characters.