In contemporary Biblical studies, there are two hypotheses to account for all the overlapping material. The one that currently holds favor with most is the Two-Source Hypothesis
, which holds that Mark and a lost document, commonly referred to as "Q", were both sources for Matthew and Luke. The minority viewpoint is the Farrer Hypothesis
, which holds that Mark was a source for both Matthew and Luke, and that Matthew was also a source for Luke. Each hypothesis has strengths and weaknesses over the other, which is why there is no consensus (at least, as of yet, anyway).
In any event, modern Biblical scholarship is now almost universally agreed, believers and skeptics alike, that we do not know who the authors actually were. There are a handful of holdouts who continue to insist that (for example) Matthew was, indeed, written by the apostle Matthew, but they're very much in the minority. In fact, there are a number of good reasons to think that it was not