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 Evangelist would seem to have spoken falsely, in saying, that before the cock crew, he would deny Him thrice. Again, if he had finished the entire threefold denial before the cock began to crow, Mark would in the person of the Lord seem to have said needlessly, “Before the cock crow twice, thou shall deny Me thrice.”
But because that threefold denial began before the first cock-crowing, the other three did not notice when Peter was to finish it, but how great it was to be, that is, threefold, and when it was to begin, that is, before the cock crew, although the whole was conceived in his mind, even before the first cock crew; but Mark has related more plainly the interval between his words themselves.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Con. Even. iii, 2