Augustine of Hippo — as excerpted in the Catena Aurea on John 18:25-27

“(de Con. Evang. iii. 6) Here we find Peter not at the gate, but at the fire, when he denies the second time: so that he must have returned after he had gone out of doors, where Matthew says he was. He did not go out, and another damsel see him on the outside, but another damsel saw him as he was rising to go out, and remarked him, and told those who were by, i. e. those who were standing with her at the fire inside the hall, This fellow also was with Jesus of Nazareth. (Matt. 26:71, 72) He heard this outside, and returned, and swore, I do not know the man. Then John continues: They said therefore unto him, Art not thou also one of His disciples? which words we suppose to have been said to him when he had come back, and was standing at the fire. And this explanation is confirmed by the fact, that besides the other damsel mentioned by Matthew and Mark in the second denial, there was another person, mentioned by Luke, who also questioned him. So John uses the plural: They said therefore unto him. And then follows the third denial: One of the servants of the high priest, being his kinsman whose ear Peter cut off, saith, Did not I see thee in the garden with Him? That Matthew and Mark speak of the party who here question Peter in the plural number, whereas Luke mentions only one, and John also, adding that that one was the kinsman of him whose ear Peter cut off, is easily explained by supposing that Matthew and Mark used the plural number by a common form of speech for the singular; or that one who had observed him most strictly put the question first, and others followed it up, and pressed Peter with more.”