Difference between Hinduism and Christianity?

Christianity is monotheistic because there is only one God. The three aspects of God are just that; aspects to one whole. In polytheism, there are many gods who are separate deities, with distinct personalities and functions. And I'm not going to cite any sources other than my own mind, because it's a widely accepted truth. This is definitely not the only difference between the two religions, but the only one you asked about specifically.