Augustine of Hippo

And it is supposed that in the second denial he was addressed by two persons, namely, by the maid whom Matthew and Mark mention, and by another whom Luke speaks of. With respect then to what Luke here relates, And after a little while, &c. Peter had already gone out of the gate, and the cock had crowed the first time, as Mark says; and now he had returned, that, as John says, he might again deny standing by the fire. Of which denial it follows, And Peter said, Man, I am not.

Catena Aurea by Aquinas ↗

Ambrose of Milan

For he preferred to deny himself rather than Christ, or because he seemed to deny being of the company of Christ, he truly denied himself.

Catena Aurea by Aquinas ↗

Bede

After a little while, another saw him and said, "You also are one of them." But Peter said, "Man, I am not." In this denial by the blessed Peter, we learn that not only is Christ denied by him who says he is not Christ, but also by him who, although he is a Christian, denies being a Christian; the Lord, however, did not say to Peter, "You will deny that you are my disciple," but "You will deny me." Therefore, he denied him when he denied being his disciple.

On the Gospel of Luke ↗