And let's factor in that the NT version of Matthew (in every manuscript and translation we have) is combined with Mark, as is Luke (which I think also used Matthew, as per the Farrer Hypothesis). Does this early harmonization not suggest that more or less all early Christians were okay with those three gospels (with a few tweaks) from the get go?
It could if there were no accounts of factions and different church branches in play in the exact time period the gospels appeared
That Paul is battling 'Judaisers' lets call them Nazarenes suggests to me they had their own gospel with theological differences and they did!
Then that as far as we know Paul never provided a gospel to his churches, may indicate those he founded initially resisted gospels completely
Mark may even be the gospel made for those churches
The NT is full of reports of these factions some obvious some not so obvious
That Matthew uses Mark could be explained by having Matthew itself be a combination of Mark and the Nazarene gospel, which I think was brought about by a merger of Pauline and Nazarene factions around that time. This resulted in some compromises theologically which led some Nazarenes to reject this and became the Ebionites. It also resulted in the doctrine of hell being adopted which was in the Nazarene gospel but not taught by Paul or in Mark (compare Paul's lack of any hell and Mark's single hell reference, likely copied from Matthew, to Matthew's large quantity of references). So this merger advanced Matthew as the primary gospel from then on
This then explains these differences in theology between Mark and Matthew and the latters more pro-Judaic stance from it's Nazarene origins
Luke is harder to explain in terms of church branches we know about and i can't offer an explanation for it's appearance
John comes from another known branch the Johannine community. This one also lacks hell and independent from the others unless some far more ancient original sources existed that even Mark used. Such a community able to produce a gospel like John can easily be imagined to not favour the others, but later on in the 2nd century the strength of the overall orthadox Christian church led to general acceptance and promotion of all four
All this i'm suggesting can only be crude approximation without knowing the exact interplay of events and goings on but i think it's a useful construct to explain what we see, what with all the splits and factions known about that involved real differences
It was exploring 'hell' that got me in to this in the first place and it's the gift that keeps on giving as far as i'm concerned as i'm pretty sure this was not an original church belief. I think there's evidence for a merger of Paul and Peter factions too, while I see the Johannine community as the one more closely connected to the original church itself than the other two although they are connected too, it was John that really knew Jesus
I do think this could help solve other problems as well basically to see the early church as divided into camps at an early date and go from there