My hypothesis

[SteveII]

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Actually the gospels only count as two sources. Matthew and Luke are clear plagarisms of Mark. [1] And all they do is document what people believed at certain periods in the late first century and second century CE, two to five generations after Yeshua's alleged death. [2]  And imperfectly at that, because we know the gospels have been subject to insetions and editorialising over the subsequent centuries which have massively changed their messages. [3] 

1. No, actually you are taking the concept that Mark existed first and was available to the other two and twisting it to fit your narrative. But thanks for pointing out that there is another source as evidence (Q) written immediately following Jesus. 

The majority view of modern scholars is that Mark was the first gospel to be composed and that Matthew (who includes some 600 of Mark's 661 verses) and Luke both drew upon it as a major source for their works.[19][20] The author of Matthew did not, however, simply copy Mark, but used it as a base, emphasising Jesus' place in the Jewish tradition and including other details not covered in Mark.[21] An additional 220 (approximately) verses, shared by Matthew and Luke but not found in Mark, from a second source, a hypothetical collection of sayings to which scholars give the name "Quelle" ("source" in the German language), or the Q source.[22] This view, known as the Two-source hypothesis (Mark and Q), allows for a further body of tradition known as "Special Matthew", or the M source, meaning material unique to Matthew; this may represent a separate source, or it may come from the author's church, or he may have composed these verses himself.[20] The author also had at his disposal the Greek scriptures, both as book-scrolls (Greek translations of Isaiah, the Psalms etc.) and in the form of "testimony collections" (collections of excerpts), and, if Papias is correct, probably oral stories of his community.[23] These sources were predominantly in Greek,[24] but mostly not from any known version of the Septuagint;[25] although a few scholars hold that some of them may have been Greek translations of older Hebrew or Aramaic sources.[26][27] https://en.wikipedia.org/wiki/Gospel_of_Matthew#Sources

The author used as his sources the gospel of Mark, the sayings collection called the Q source, and a collection of material called the L (for Luke) source.[4] Mark, written around 70 AD, provided the narrative outline, but Mark contains comparatively little of Jesus' teachings.[19] For these Luke turned to Q, which consisted mostly, although not exclusively, of "sayings".[20] (Most scholars are reasonably sure that Q existed and that it can be reconstructed).[21] Mark and Q account for about 64% of Luke. The remaining material, known as the L source, is of unknown origin and date.[22] Most Q and L-source material is grouped in two clusters, Luke 6:17-8:3 and 9:51-18:14, and L-source material forms the first two section of the gospel (the preface and infancy and childhood narratives).[23] https://en.wikipedia.org/wiki/Gospel_of_...nd_setting

2. The Gospels were written, at most, 60 years after Jesus. Most have a date range earlier than that--still within the lifetime of witnesses. Two points:
A. The Gospels were written by editors who wanted to get the things written down that the Matthew, Mark, and John communities of believers believed to be true up until that point. 
B. The fact that the epistles (which started around 50 AD) referenced the core teachings (Jesus' message, death, resurrection) proves that the contemporaries of all the eyewitnesses already believed the contents we later find in the Gospels. So, I we are not simply using the Gospels as evidence--it is a package deal. 

3. That assertion will require some details or links, because I don't believe "we know..." anything of the sort.