The apostle John is the author of this epistle as with 1 and 2 John. In both 2 and 3 John the author identifies himself as “the elder.” Also, note the similarities found in both epistles: “love in the truth” (v. 1 of both letters) and “walking in the truth” (v. 4 of both letters). The style of both epistles are clearly the same, and efforts to deny that John is the author of all three epistles has no real support or evidence.
The ancient opinion that the Apostle John wrote this letter, as well as the other two, may be readily accepted. The arguments that support apostolic authorship of 1 John carry over to this tiny epistle by virtue of the clear stylistic ties. Moreover, the self-confident authority of the writer of 3 John (cf. v. 10) also befits an apostle.
This is clearly the most personal letter of John. It is addressed to a man John called “the beloved Gaius” (v. 1) regarding ecclesiastical problems Gaius was facing. The recipient is simply identified no further than by the above description which suggests he was well known by those of the churches of Asia Minor where John served for the last years of his life. Gaius is a familiar name in the New Testament. It appears in Romans 16:23 (a Gaius of Corinth), Acts 19:29 (a Gaius of Macedonia) and Acts 20:4 (a Gaius of Derbe).
Again, the similarities between 1 and 2 John suggest a similar date of somewhere between A.D. 85-90.
John writes Gaius regarding the issue of hospitality and physical support to itinerate Christian workers (missionaries), especially when they were strangers. The theme centers around the contrast between the ministry of Gaius and his generous demonstration of Christian love as one walking in the truth in contrast to the behavior of the selfishness of Diotrephes who, rather than walking in the truth, rejected what John had said and was seeking personal preeminence (v. 9).
Several distinct purposes emerge in this epistle: (1) to commend Gaius (vv. 1-6a), (2) to instruct and encourage the continuation of his support for the Christian workers John had evidently sent (vv. 6b-8), (3) to rebuke Diotrephes for his self-centered behavior (vv. 9-11), (4) to give instruction for Demetrius (v. 12), and (5) to inform Gaius of John’s desire and intention to visit and deal with the difficulties (vv. 10a, 13-14).
While no one word stands out as in 2 John by way of repetition, the key idea is faithful ministry of selfless service to others as fellow workers in the truth (vv. 5-8).
As in 2 John this is not applicable with only one chapter.
While the name of Jesus Christ is not mentioned directly, He is referred to in the statement, “For they have gone forth on behalf of ‘The Name.’” This is undoubtedly a reference to ministry on behalf of the Lord Jesus (see Acts 5:40-41 where we have the identical Greek construction in v. 41). Paul uses a similar phrase in Romans 1:5, and in 1 John 2:12 the author wrote, “your sins are forgiven on account of His (Christ’s) name.” John’s Gospel also makes reference to believing “in the name of Jesus” (John 1:12, 3:18).