John Chrysostom

For since the voice that said, "This is my beloved Son," would seem to the multitude rather to belong to John, for it added not, "This that is baptized," but simply "This," and every hearer would conceive it to be said concerning the baptizer, rather than the baptized, partly on account of the Baptist's own dignity, partly for all that hath been mentioned; the Spirit came in form of a dove, drawing the voice towards Jesus, and making it evident to all, that "This" was not spoken of John that baptized, but of Jesus who was baptized. …

Homily on the Gospel of Matthew 12 ↗

Augustine of Hippo

Not as before by Moses and the Prophets, neither in type or figure did the Father teach that the Son should come, but openly showed Him to be already come, This is my Son. (de Trin. iv. 21.) Here are deeds of the whole Trinity. In their own substance indeed Father, Son, and Holy Spirit are One without interval of either place or time; but in my mouth they are three separate words, and cannot be pronounced at the same time, and in written letters they fill each their several places. …

Catena Aurea by Aquinas ↗

Ambrose of Milan

And no wonder that the mystery of the Trinity is not wanting to the Lord's laver, when even our laver contains the sacrament of the Trinity. The Lord willed to show in His own case what He was after to ordain for men.

Catena Aurea by Aquinas ↗

Jerome

And behold, the heavens were opened for him, and he saw the Spirit of God descending like a dove and coming upon him. And behold, a voice from heaven, saying: 'This is my beloved Son, in whom I am well pleased.' The mystery of the Trinity is demonstrated in the baptism: the Lord is baptized, the Spirit descends in the form of a dove, and the voice of the Father, testifying to the Son, is heard. …

Commentary on Matthew ↗