42 In due order, let us now inquire how it is that Mark, after telling us that they parted His garments when they were crucifying Him, casting lots upon them what every man should take, has appended this statement, “And it was the third hour, and they crucified Him.” Now here he had already made the declaration, “And crucifying Him, they parted His garments;” and the other evangelists also certify that, when He was crucified, they parted His garments. If, therefore, it was Mark's design to specify the time at which the incident took place, it would have been enough for him to say simply, “And it was the third hour.”
What reason, then, can be assigned for his having added these words, “And they crucified Him,” but that, under the summary statement thus inserted, he intended significantly to suggest something which might be found a subject for consideration, when the Scripture in question was read in times in which the whole Church knew perfectly well what hour it was at which the Lord was hanged upon the tree, and the means were possessed for either correcting the writer's error or confuting his want of truth?
But, inasmuch as he was quite aware of the fact that the Lord was suspended [on the cross] by the soldiers, and not by the Jews, as John most plainly affirms, his hidden object [in bringing in the said clause] was to convey the idea that those parties who cried out that He should be crucified were the Lord's real crucifiers, rather than the men who simply discharged their service to their chief in accordance with their duty. We understand, accordingly, that it was the third hour when the Jews cried out that the Lord should be crucified.
And thus it is intimated most truly that these persons did really crucify Christ at the time when they cried out. All the more, too, did this merit notice, because they were unwilling to have the appearance of having done the deed themselves, and with that view delivered Him up unto Pilate, as their words indicate clearly enough in the report given by John. For, after stating how Pilate said to them, “What accusation bring ye against this man?” his version proceeds thus: “They answered and said unto him, If he were not a malefactor, we would not have delivered him up unto you.
Then said Pilate unto them, Take ye him, and judge him according to your law. The Jews therefore said unto him, It is not lawful for us to put any man to death.” Consequently, what they were especially unwilling to have the appearance of doing, that Mark here shows that they actually did do at the third hour. For he judged most truly that the Lord's murderer was rather the tongue of the Jews than the hand of the soldiers.
Source: The Harmony of the Gospels (New Advent)