25 Accordingly, let us next examine into the consistency of the evangelists so far as the third denial is concerned, which we have previously instanced in the statement given by Matthew only. Mark then goes on with his version in these terms: “And a little after, they that stood by said again to Peter, Surely you are one of them; for you are a Galilæan. But he began to curse and to swear, saying, I know not this man of whom you speak. And immediately the second time the cock crew.” Luke, again, continues his narrative, relating the same incident in this fashion: “And about the space of one hour after, another confidently affirmed, Of a truth this fellow also was with him; for he is a Galilæan.
And Peter said, Man, I know not what you say. And immediately while he yet spoke the cock crew.” John follows with his account of Peter's third denial, which is thus given: “One of the servants of the high priest, being his kinsman whose ear Peter cut off, says, Did not I see you in the garden with him? Peter then denied again; and immediately the cock crew.” Now what precise period of time is meant under the phrase, “a little after,” which is employed by Matthew and Mark, is made clear by Luke, when he says, “And about the space of one hour after.”
John, however, conveys no intimation of this space of time. Again, with respect to the circumstance that Matthew and Mark use the plural number instead of the singular, and speak of the persons who were engaged with Peter, while Luke mentions only a single individual, and John, too, specifies but one, particularizing him further as kinsman to him whose ear Peter cut off; we may easily explain it either by understanding Matthew and Mark to have adopted a familiar method of speech here in employing the plural number simply instead of the singular, or by supposing that one of the persons present— one who knew Peter and had seen him— took the lead in making the declaration, and that the rest, imitating his confidence, joined him in pressing the assertion upon Peter.
If this is the case, then two of the evangelists have given the general statement, using simply the plural number; while the other two have preferred to particularize only the one special individual who played the chief part in the transaction. But, once more, Matthew affirms that the words, “Surely you also are one of them, for your speech betrays you,” were spoken to Peter himself. In like manner, John tells us that the question, “Did not I see you in the garden with him?” was addressed directly to Peter.
But Mark, on the other hand, gives us to understand that the sentence, “Surely he is one of them, for he is also a Galilæan,” was what those who stood by said to each other about Peter. And, in the same way, Luke indicates that the declaration uttered by the other person, who said, “Of a truth, this fellow also was with him, for he is a Galilæan,” was not addressed to Peter, but was made regarding Peter. These variations, however, may be explained either by understanding the evangelists, who speak of Peter as the person directly addressed, to have fairly reproduced the general sense, inasmuch as what was spoken about the man in his own presence was much the same as if it had been spoken immediately to him; or by supposing that both these methods of address were actually practised, and that the one has been noticed by the former evangelists, and the other by the latter.
Moreover, we take the second cockcrowing to have occurred after the third denial, as Mark has expressly informed us.
Source: The Harmony of the Gospels (New Advent)