If you would like to explore this I would suggest reading:
A History of God: The 4,000-Year Quest of Judaism, Christianity and Islam Paperback – August 9, 1994
by Karen Armstrong (Author)

Yep, I've always been confused by that Holy Trinity stuff. But like the rest of the Bible, it doesn't make a lot of sense. I think some literate people were eating some particular fungi back in the day...

I agree with the previous posts on this one, it derived from other religions, a real Heinz 57. Yet I say monotheism because it is one religion, even though the divinity has multiple personality disorder.

The Christian religion has not been founded, as somebody wants us to believe, by Jesus Christ (that I have doubt he ever existed) but by Saul of Tarsus (Paul the Apostle). Saul was an intellectual and very wise person. He saw a potential on some ideas of some Hebraic cult (like the Nazarenes) and saw there a potential of a new religion. He modeled this new religion, not under the Judaic religion but, as it was his culture, from the Greek religion, taking something from Judaism. So yes it is monotheistic but he needed something to substitute the several gods, and from this come the Saints. The Catholic religion has one saint as protector for any thing.
Conclusion: Christianity it is a minestrone soup religion.

Father, son and the Holy spirit. That's trinity.