Luke 22:34

‘And he said, “I tell you, Peter, the cock shall not crow this day, until you will three times deny that you know me.” '

But Jesus tenderly turned to him and warned him of what was to come. Note the change from ‘Simon' to ‘Peter' (a rock). This is the only time that we know of that He has actually directly addressed him as Peter, although it was He Who gave him the name (Mark 3:16), and had promised that one day he would provide the rock on which the new people of God would be founded, the declaration of Jesus as the Christ (Messiah) (Matthew 16:18). Rock man he may think himself to be, He says, but let him realise that before cockcrow he would deny Him three times.

There is no contradiction between this and Mark's reference to the cock crowing twice. Luke is speaking of cockcrow in general. He does not want to puzzle his readers by speaking of a double cock crow. The third of the Roman watches was called ‘cockcrow', ending around 3:00 am. But Mark and Jesus were aware of the reality of life known to them through their familiarity with Jerusalem, and that the distant cocks would be heard first across the valley, and the nearer cocks a short time afterwards. In Jerusalem cock crow would only come after the second crowing of the cocks was heard.

It will be noted that in Matthew and Mark similar words as these were spoken as they were approaching the Garden. It may well have been that Jesus gave this warning twice, for the contexts and the wording are quite different. Or it may be that Luke (or his source) has transferred it here so as to fit in with his chiasmatic scheme.