82 But another difficulty in the way of this settlement arises here. For, when John has recorded how the Lord was seen, not by the eleven on the mountain, but by seven of them when they were fishing in the sea of Tiberias, he appends the following statement: “This is now the third time that Jesus showed Himself to His disciples, after that He was risen from the dead.” Now, if we accept the theory that the Lord was seen by the company of the eleven disciples within the period of these eight days, and previous to His being seen by Thomas, this scene by the sea of Tiberias will not be the third but the fourth time that He showed Himself.
Here, indeed, we must take care not to let any one suppose that, in speaking of the third time, John meant that there were in all only three appearances of the Lord. On the contrary, we must understand him to refer to the number of the days, and not to the number of the manifestations themselves; and, further, it is to be observed that these days are not presented as coming in immediate succession after each other, but as separated by intervals in accordance with intimations given by the evangelist himself.
For, keeping out of view His appearance to the women, it is made perfectly plain in the Gospel that He showed Himself three several times on the first day after He was risen; namely, once to Peter; again to those two disciples, of whom Cleophas was one; and a third time to the larger body, while they were conversing with each other as the night came on. But all these John, looking to the fact that they took place on a single day, reckons as one appearance. Then he identifies a second— that is to say, an appearance on another day— with the occasion on which Thomas also saw Him; and he particularizes a third by the sea of Tiberias, that is to say, not literally His third appearance, but the third day of His self-manifestations.
Thus the result is, that after all these incidents, we are constrained to suppose this other occasion to have occurred on which, according to Matthew, the eleven disciples saw Him on the mountain in Galilee, to which He had gone before them according to appointment, so that all that had been foretold, both by the angel and by Himself, should be fulfilled even to the letter.
Source: The Harmony of the Gospels (New Advent)