As Kummel states (Introduction to the New Testament, p. 446): "No other NT letter, not even Phlm has so completely the form of a Hellenistic private letter as II and III John. Both are real letters." Kummel also states (op. cit., p. 449): "II and III John were written by the same author. They use the same language, they agree in their length and in their epistolary form (address, introduction, conclusion). In the heading they carry the same distinctive self-designation: ο πρεσβυτερος."
Concerning this title, Kummel writes (op. cit., p. 451): "There are therefore two possibilities: either a man unknown to us who was perhaps a member of the presbyterion (cf. I Tim 4:14) bore this title in some special sense without our being able to know anything more certain about him, or ο πρεσβυτερος is a reference to the fact that this man belonged to the circle of 'those presbyters' whom Papias and Irenaeus and Clement present as guardians and bearers of the apostolic tradition." The question of whether this author is to be identified with the presbyter John mentioned by Papias cannot be answered with any certainty.
The earliest attestation for II John comes from Irenaeus, and the Muratorian Canon accepts two letters of John only. Because the author appears to stand in the Johannine tradition that is thought to have flourished in the first quarter of the second century, II and III John have been dated to the same time period.