Contradiction with Mark 14:66-68
John 18:17 records Peter's first denial of Jesus to a damsel at the door, while Mark 14:66-68 suggests a different account mentioning Peter's denial to a servant girl by the fire.
Mark 14:66-68: And as Peter was beneath in the palace, there cometh one of the maids of the high priest:
Contradiction with Matthew 26:69-70
John 18:17 portrays Peter's first denial at the entrance to the courtyard, whereas Matthew 26:69-70 states the encounter occurred elsewhere.
Matthew 26:69-70: But he denied before [them] all, saying, I know not what thou sayest.
Contradiction with Luke 22:55-57
John 18:17 describes Peter's denial to a maid at the door, but Luke 22:55-57 has the denial taking place while Peter is sitting by the fire warming himself.
Luke 22:55-57: And when they had kindled a fire in the midst of the hall, and were set down together, Peter sat down among them.