The book of Acts mentions Luke's gospel, so it was obviously written after it. Portions of Luke and Matthew are written word for word, and Matthew's was probably written first. There was also a very strong oral tradition in those days, and that could possibly account for the two following word for word in several sections of text. There is a third, rather weird, theory that says that both Matthew and Luke copied from a third non-existent document that no one has ever seen. The people who like this little speculation call the mysterious third document, "Q." So much for theology.