Boiling down the Synoptic Problem as much as possible, you have:
Mark: outline of Jesus’s ministry, with a limited selection of teachings. Passion story #1. (“Passion” here is a technical term used to group together accounts of the Last Supper, arrest, crucifixion, and accounts of Resurrection appearances.)
Matthew: Nativity story, told from Joseph’s perspective; account of Jesus’s ministry following Mark closely, including identical or extremely similar phrasings; Jesus’s teachings greatly expanded, with the majority of them in five main apparent sermons. Passion story #2.
Luke: Extensive Nativity and pre-Nativity accounts, told from Mary’s perspective; account of Jesus’s ministry following Mark fairly closely; Jesus’s teachings greatly expanded, including much of Matthew’s supplemental material, but in a very different placement of them, and sometimes with them carrying different points, than Matthew had had; Passion story #3.
Obviously some borrowing has been going on – but from whom by whom, and with the differences having what purpose, is hotly disputed. It’s pretty clear and consensus opinion that Luke used Mark, along with a lot of other material – he as much as says so in the dedication at the beginning of his book. The Q source, which is purely hypothetical, is one way in which scholars explain how Matthew and Luke have Jesus giving the same teachings, but at different times and places.
As for the overall idea – there is no question, even among conservative evangelicals, that Jesus is a myth in the Campbellian sense – someone whose life story is meaningful on a much higher plane than a random obituary from today’s paper. The question of what historically underlies that myth is what is usually argued when the “myth” allegation gets flung around. Some excellent discussion on that has already been done in this thread – kudos to the expositors!
My bottom line on this is: Setting to one side the religious implications of his story for the moment, Jesus as a person is about as well historically documented as the average “significant” individual of classical times. The parallel to Socrates is often advanced, and probably very much on point: Most of what we know of both men comes from the writings of followers, who are well known to have had particular axes to grind and points to make in what they wrote about them. But each set of accounts references an actual historical figure, who is known aside from their accounts only from a few scattered references, but who, thanks to their accounts, becomes a living, breathing, knowable human being.