I had no idea that Trinitarian doctrine - which is an odd fit with a religion which claims a succession from Judaism - has such a rich history.
The Sumerian religion has a trinity of leading gods, representing the heavens, earth and the watery depths: Anu, Enlil and Enki.
The Babylonians succeeded to the Sumerian vision and used the equilateral triangle to symbollise their trinity, just as the Church ended up doing.
The Egyptians had two trinities: a father, mother and son grouping via Isis, Serapis and Horus; and through Amun, a face, a body and a spirit three-for-the-price-of-one model - "All Gods are three: Amun, Re, Ptah; they have no equal. His name is hidden as Amun, he is Re... before [men], and his body is Ptah." (Hornung 219).
Though maybe it is fitting, as the Bible's first book of Genesis owes a great deal to Babylonian ritual. The Bible's authors were surrounded by Babylonian culture when they gathered together their text: so is it any wonder that Yahweh's earth issues from the watery deep which is the abode of Enki.
The father, the son and the holy spirit. Amen.
Oh and Amen is a corruption of the name of the Egyptian god Amun.
Here's more: http://www.heraldmag.org/olb/Contents/doctrine/The%20Origin%20of%20the%20Trinity.htm