When the canon of the New Testament was put together, every single apostle church
the Hellenistic church of Paul contributed a gospel, letters and sometimes other scriptures. The criteria of incorporation into the canon are that there must be a balanced nature of the origin from each of the four churches and pleas for the unity of the Christian church.
Therefore the gospels according to Matthew, Mark, Luke and John were incorporated into the canon, along with the Acts of the Apostles, the Revelation and the collections of letters of each of the churches.
The Letter of Paul to Philemon and the Third Letter of John belonged, and still belong, to the canon because they plead for the unity of the church as a whole regarding social classes and the creed of faith.