12 Luke, on the other hand, has omitted to mention the number of times that He prayed. He has told us, however, a fact which is not recorded by the others— namely, that when He prayed He was strengthened by an angel, and that, as He prayed more earnestly, He had a bloody sweat, with drops falling down to the ground. Thus it appears that when he makes the statement, “And when He rose up from prayer, and had come to His disciples,” he does not indicate how often He had prayed by that time. But still, in so doing, he does not stand in any kind of antagonism to the other two. Moreover, John does indeed mention how He entered into the garden along with His disciples. But he does not relate how He was occupied there up to the period when His betrayer came in along with the Jews to apprehend Him.
Source: The Harmony of the Gospels (New Advent)