In the traditional nativity story we all have in our heads, the accounts in Matthew and Luke (there are none in Mark or John) are combined. Each is seen as relating parts of the whole, and we need to add them together to form the story we celebrate every Christmas. We tend also to include things not in the Gospels at all, but derived from later traditions: the ox and ass (borrowed from Isaiah 1:3), the donkey that Mary rode on and the magi (not even numbered in the Bible) as three kings called Caspar, Melchior and Balthazar.