Simple: Matthew the disciple took notes, and wrote them up under the guidance of other apostles - James (who wrote the epistle) has some similar content. The original source were the apostles themselves, the same with Luke, except much of his material is supplied by the named women, hence the differences between Luke and Matthew. Mark weaved Mt and Lk together, adding material from his other source.
Just non-sense and that's not just my view but the views of the overwhelming majority of expert scholars for over 100 years.
Let's just think this through Spud.
Imagine is your notion was correct and Mark, when writing his gospel, had Matthew and Luke in front of him. So he's a bit worried about the cost of papyrus so wants to trim some stuff, presumably things he feels aren't really important.
So - Sermon on the mount - nope, in the bin - we need space for pig stories
Lord's prayer - hmm, can't see that being important - get rid of that.
Nativity stories - irrelevant to christianity - gone
Post-resurrection appearances - nope can't see how those are important to christianity - in the bid.
Great plenty of space for the pigs.
I simply beggars belief that he would have done that - the only conclusion is that Mark didn't have to hand knowledge of the Sermon on the Mount, the lords prayer, the nativity or the post-resurrection appearances or they would have undoubtedly been in his gospel. Or if he was aware of them (perhaps some knowledge of the Q tradition and Paul), he didn't believe them.