Well, P4I, it's a very hard issue to get around coming from a Jewish background - that God is only one (and that he is trancendental). And then some Christians also say God really IS three, living in a relationship of sorts. Others disagree.
Jesus himself said confusing things about it - for example he claimed to be God but he also claimed that he only did what his "father" told him. He also spoke of the Spirit as someone else ("he will take from what is mine," etc. etc.) and as himself "I will not leave you alone, I will come to you."
I think therefore - personally - the deal is, the trinity is a way of expressing God and how he interacts with us in as simple a way as is possible. Not perfectly accurate, but something to hold onto.
So for those who disbelieve in the Trinity on biblical grounds, while I disagree with the view, I can see where they came to their conclusions.
Now if it's simply because the Talmud and the rabbis say God can't manifest... then I have an issue with it, since that is pretty contradictory to the scriptures. (and iirc wasn't a belief until folks wanted to refute christianity.)