Hi all!
We (orthodox Jews) categorically reject the notion of a triune God as conceptually and semantically ill-disguised tritheism. Our great 9th century CE sage, Saadya Gaon (who lived in what is now Iraq, see
http://tinyurl.com/3yjtd) said that to believe that God is triune is to define & limit Him by the physical concepts of quantity and number. To believe in a truly transcendant God (that is who transcends all physical constructs/concepts, including those of quantity & number), who is Wholly Other, one must believe that He is One. Since He created all things corporeal, He Himself cannot be, or have been at one time, corporeal; to believe otherwise, Saadya Gaon taught, is to define Him by, & limit Him to, the corporeal form that he presumably chose. I have heard many Christians use the analogy that the three "persons" of the trinity are all one the same way that I, ferinstance, am a father to my boys, a husband to my wife, a son to my parents & a brother to my brother. I would reply that this is a human analogy & as such, it cannot be applied to a transcendant, wholly other, God. The same goes for the ice-water-steam analogy. H20 is a created, physical, corporeal thing & as such, cannot be applied to a transcendant, wholly other, God. The uniqueness of One is that it presumes no self-division. Like the smallest whole number that it, in fact, is, it cannot be broken down into constituent parts. Whereas 3 = 1 + 1+ 1, 1 (simply) = 1; it is in this sense that 1 is transcendant in a way that 3 is not. We believe that the Oneness of God is unbounded both internally & externally.
My personal view is that the early Christians found the (originally) Jewish concept of a transcendant, wholly other God too difficult to grasp and recast Him in our image, i.e. they made Him flesh & gave Him our form and made Him much easier to relate to. It seems to me that this is paganism with a (superficial) non-pagan veneer. (I mean no offense to anyone, God forbid!)
Be well!
ssv