3 John in Wikipedia

The Third Epistle of John, usually referred to simply as Third John and often written 3 John, is a book of the New Testament attributed to John the Evangelist, traditionally thought to be the author of the Gospel of John and the other two epistles of John. This Epistle is the shortest book in the Bible. Indications within the letter suggest a genuine private letter, composed to Gaius to commend a party of Christians led by Demetrius, who were strangers to the place where he lived, and who had gone on a mission to preach the gospel (verse 7). The purpose of the letter is to encourage and strengthen Gaius, and to warn him against the party headed by Diotrephes, who refuses to cooperate with the presbyteros who is writing...

Link: https://en.wikipedia.org/wiki/3_John