Areas for research:
Zoroastrianism (preChristian monotheism)
Judiasm (preChristian monotheism)
Hinduism (preChristian monotheism)
Gnostics (preChristian monotheism)
It\'s also of note that Vishnu has 10 manifestations (We are in the age of Kali presently for instance, I believe. The 9th manifestation of Vishnu, with a 10th to come who will bring about the end of the world and judgement to everyone. Hindu didn\'t just do three in one, they did ten in one.

If you want to explore the numerology of it, research the works of Jewish Mystic scholars. Reading over Revelations of the New Testament might lead you to some interesting things as well.
Edit: In answer to your question, part of the problem is that numerology is a matter of mysticism. It isn\'t taken entirely seriously by the academic community, to my understanding. There is a lot of literature to it though. If you want to look at other forms of numerology for comparison, you might want to look at Roman architecture, fibonacci sequences, golden ratios, ratios in the paintings and works of Da Vinci, physics constants of the universe (there\'s some 25 of them and they are all EXACTLY right to support life, according to some).