It is my understanding that there are common elements in Mark, Matthew and Luke that are thought to be attributable to a common text. I think the text is referred to as “Q”, which would have been preparred during or immediately after Christ’s death
I thought the "Q" tradition was supposed to be things common to Matthew and Luke but not Mark, as Mark is viewed as a source for both Matthew and Luke but there are additional items in both that do not appear in Mark.
That is a common theory. But I find it interesting that none of the second and third century Church fathers mention this Q gospel but all quote extensively from the cannonical Gospels. I figured they would have heard of this Q text and mentioned it if it existed.