How many times have you heard someone say "The Bible says" and then follow that up with some drivel that you couldn't even find in The Message?
When it comes to the Nativity accounts in Matthew and Luke, many Christians assume certain details are biblical when in fact they are not.
The Bible does not specify the number of Magi. The tradition of 3 wise men probably arose from the number of gifts presented to Jesus. Nor are the Magi specified to be kings. The Magi were most likely educated men who counseled kings. The counselors in Daniel 2 come to mind. A further note on the Magi--they are not named. Balthazar, Melchior, and Caspar are extra-biblical traditions.
The Bible does not specify that Jesus was born in a barn (stable). Some suspect he was actually born in a cave, a more likely environment in first century Judea where animals were kept. Animals aren't mentioned as present at the birth, either, but Jesus was laid in a manger, so possibly animals were present.
The innkeeper never appears in the biblical account. All we know is that there was no room for them in the inn (Lk 2:7). How they found this out is a matter of speculation.
The Bible does record that Joseph and Mary traveled from Nazareth to Bethlehem, but it does not say that Mary rode a donkey. Perhaps she did.
Luke does not specify that the angels sang to the shepherds. All the text says is that they were "praising God, saying, 'Glory to God in the highest.'" Perhaps they were singing, though.
Many people understand Mary to be a teen and Joseph to have been quite bit older, but the Bible does not say. (It could nonetheless be true.) What we know is that Mary was a virgin when Jesus was conceived, and she remained so until he was born (Mt 1:25). However, contra Roman Catholicism, she did not remain a virgin (again see Mt 1:25), for she later gave birth to daughters and at least 4 other sons (Mk 6:3).
Elizabeth's age, however, is commented on. She is "well along in years" (Lk 1:7), or "in her old age" (Lk 1:36).
So what is in the Nativity accounts? Read them in Mt 1-2 and Lk 1-2.