The epistles of John.

Three canonical catholic epistles.

Attributed author(s).

Text(s) available.
On site:

Epistle 1 of John 1-5 (on site, Greek only).
Epistle 2 of John 1 (on site, Greek only).
Epistle 3 of John 1 (on site, Greek only).
Online Greek Bible (Greek only).
Bible Gateway (English only).
HTML Bible:
1 John 1, 2, 3, 4, 5 (Greek and English).
2 John 1 (Greek and English).
3 John 1 (Greek and English).
HTML Bible:
1 John 1, 2, 3, 4, 5 (Latin Vulgate only).
2 John 1 (Latin Vulgate only).
3 John 1 (Latin Vulgate only).
Zhubert (Greek and English).
Kata Pi:
1 John 1, 2, 3, 4, 5 (Greek and English).
2 John 1 (Greek and English).
3 John 1 (Greek and English).
Sacred Texts:
1 John 1, 2, 3, 4, 5 (polyglot).
2 John 1 (polyglot).
3 John 1 (polyglot).

Useful links.
Johannine literature at Catholic Resources.
1 John, 2 John, and 3 John at Early Christian Writings.
1 John, 2 John, and 3 John by Daniel Wallace.
1, 2, and 3 John in the Catholic Encyclopedia.
Johannine epistles at Kata Pi (R. M. Grant).
The Sitz im Leben of Third John: A New Reconstruction (Robert M. Price).
ECW e-Catena:

1 John 1, 2, 3, 4, 5.
2 John 1.
3 John 1.

Three epistles are extant which are attributed to John, though it must be admitted that it is not always clear to which John they are being assigned. The usual candidates are John the son of Zebedee, a certain John the elder mentioned by Papias, and John the seer of the apocalypse. The epistles name their author only by title, not in in their actual text.

Polycarp to the Philippians 7.1:

Πας γαρ ος αν μη ομολογη Ιησουν Χριστον εν σαρκι εληλυθεναι αντιχριστος εστιν· και ος αν μη ομολογη το μαρτυριον του σταυρου εκ του διαβολου εστιν· και ος αν μεθοδευη τα λογια του κυριου προς τας ιδιας επιθυμιας, και λεγει μητε αναστασιν μητε κρισιν, ουτος πρωτοτοκος εστι του Σατανα.

For whoever does not confess that Jesus Christ has come in the flesh is antichrist. And whoever does not confess the testimony of the cross is of the devil. And whoever perverts the oracles of the Lord to his own lusts, and says that there is neither a resurrection nor a judgment, this man is the firstborn of Satan.

Confer 1 John 4.2-3.

Jerome, commentary on the epistle of Paul to the Galatians (translation slightly modified from Orchard and Riley, The Order of the Synoptics, pages 207-208):

Beatus Ioannes evangelista cum Ephesi moraretur usque ad ultimam senectutem et vix inter discipulorum manus ad ecclesiam deferretur nec posset in plura vocem verba contexere, nihil aliud per singulas solebat proferri collectas, nisi hoc: Filioli, diligite alterutrum. tandem discipuli et fratres qui aderant, taedio affecti quod eadem semper audirent, dixerunt: Magister, quare semper hoc loqueris? qui respondit dignam Ioanne sententiam: Quia praeceptum domini est, et si solum fiat, sufficit. hoc propter praesens apostoli mandatum: Operemur bonum ad omnes; maxime autem ad domesticos fidei.

Blessed John the evangelist, while dwelling at Ephesus, would even in extreme old age be borne with difficulty in the hands of his disciples into the church. He had not the strength for many words, yet he was in the habit of saying nothing in each of his prayers but this: Little children, love one another.1 At last the disciples and brethren who were present, irritated at always hearing the same thing, said: Master, why do you keep on saying this? His answer was wholly characteristic of John: Because it is the command of the Lord, and, if it were that alone, it would be enough. Hence this present command of the apostle: Let us do good to all, but especially to members of the household of the faith.1

1 For loving one another refer to John 13.34; 15.12, 17; 1 John 3.11, 23; 4.7, 11-12; 2 John 1.5. Confer Romans 13.8; 1 Thessalonians 4.9; 1 Peter 1.22. For little children refer to John 13.33; 1 John 2.1, 12, 28; 3.7, 18; 4.4; 5.21.
2 Refer to Galatians 6.10.