The Problem Never Completely Resolves
We cannot say much about what exactly the sources looked like that preceded the synoptic Gospels, only that that direct dependence does play a large part in the end result. Even if we do conclude that Mark is the earlier written Gospel it seems that Mark is still somewhat secondary to earlier sources. Mk 13 for instance looks to be older material very unlikely to have been freshly penned by the Gospel author. Also I have previously indicated my own theorizing that most of the healing stories apparently predate our written Gospels, although in these cases they would likely have existed only in oral form. So Mark is not necessarily the oldest 'source' of shared material.

So students should be aware that hypothesizing a chronology for dating Matthew, Mark and Luke still does not completely (re)solve the problem of Gospel sources. It merely gives us a simplified ‘working hypothesis’ for supposing how a particular Gospel author may have put their own stamp on the material which we suppose to have been already available to the author (and in many cases material likely already known by the audience).

I'm perpetually agnostic concerning 'Q.' How is it that the material in Matthew and Luke concerning John the Baptizer is written virtually word-for-word? This would be the result of someone copying slavishly from a written source. So if the text in Matthew is not being copied here into Luke (or theoretically from Luke into Matthew) whether by a secondary Gospel author (or inserted by a scribe within the first hundred years of copying) we must suppose that both have here accessed the same written source concerning John the Baptizer. Yet the other shared material in Matthew and Luke (and not found in Mark) is less likely to be from the same shared written source since everywhere else the doubly-shared material in Mt-Lk is phrased independently by both authors making it impossible to know much about the immediate source of such traditions or whether these traditions originated from the very same source as the John-Baptizer material. How can we conclude the same way (all stems from a singular written source?) with this differing evidence?

So the precise sources of all the triply and doubly shared traditions are still largely unknown even after having ‘solved’ the problem of the likely order the Gospels were published. There will always remain the problem of whether an earlier version of Mark, or Matthew or Luke was known or accessed by any of the other Gospel authors (aurally or in written form) on top of the problem of other pre-Gospel traditions.