49 Moreover, I request and admonish those who read the statement which, with the help of the Lord, has thus been elaborated by us, to bear in mind this discourse, which I have thought it needful to introduce in the present connection, in every similar difficulty which may be raised in such inquiries, so that there may be no necessity for repeating the same thing over and over again. Besides, any one who is willing to clear himself of the hardness of impiety, and to give his attention to the subject, will easily perceive how opportune the place is in which Mark has inserted this notice of the third hour, so that every one may there be led to bethink himself of an hour at which the Jews really crucified the Lord, although they sought to transfer the burden of the crime to the Romans, whether to the leaders among them or to the soldiers, [as we see] when we come here upon the record of what was done by the soldiers in the discharge of their duty.
For this writer says here, “And crucifying Him, they parted His garments, casting lots upon them, what every man should take.” And to whom can this refer but to the soldiers, as is made manifest in John's narrative? Thus, lest any one should leave the Jews out of account, and make the conception of so great a crime lie against those soldiers, Mark gives us here the statement, “And it was the third hour, and they crucified Him,”— his object being to have those Jews rather discovered to be the real crucifiers, who will be found by the careful investigator in a position making it quite possible for them to have cried out for the Lord's crucifixion at the third hour, while he observes that what was done by the soldiers took place at the sixth hour.
Source: The Harmony of the Gospels (New Advent)