Augustine of Hippo

Though all the Evangelists say that the Lord foretold that Peter was to deny before the cock crew, Mark alone has related it more minutely, wherefore some from inattention suppose that he does not agree with the others. For the whole of Peter's denial is threefold; if it had begun altogether after the cock crew, the other three Evangelists would seem to have spoken falsely, in saying, that before the cock crew, he would deny him thrice. …

Catena Aurea by Aquinas ↗

Bede

And Jesus said to him: Truly, I say to you that today, in this very night, before the rooster crows twice, you will deny me three times. And Peter, in the fervor of his faith, made a promise, and the Savior, as God, knew the future. And note that Peter denies in the night, and denies three times. But after the rooster crowed, and as the darkness lessened, the approaching light was announced, he turned and wept bitterly, washing away the filth of the threefold denial with his tears. …

On the Gospel of Mark ↗