I was once a student of comparative mythology, so the notion that some family legends might spring from the unconscious mind appeals to me.
The legend of three brothers emigrating together to America and then splitting up to settle in different parts of the country is a common myth. Some ascribe the myth to genealogical laziness, but it may have roots in the "three brothers" theme found in medieval folk tales, "in which an aged king sends his sons on a quest for some magic, rejuvenating water in a distant land." Professor E. Washburn Hopkins traced the story back to "Fountain of Youth" stories told in ancient Persia, Ireland, and elsewhere.
The Persian version substitutes for three brothers two brothers and a sister; the Keltic version turns all three into girls. Elsewhere the three are brothers, the trio still preserved, perhaps, in the numerous American families (of eight or nine generations) who independently trace their origin to "three brothers who came to America in the seventeenth century to seek their fortune." How widespread this myth is, may easily be learned from casual inquiry. I once sat at table with half a dozen unrelated people, four of whom stated that this was their "family legend." Of the four, three admitted that it was a legend without historical foundation, "a myth"; one insisted that it was certain. [Link]