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...

Dr Richard Carrier, atheist, Jesus Mythicist, once said Christianity is a complex mixture of prophecy (from pre Jesus period) history, some real and lot bad, theology and mythology, and teasing out the truth and facts require great intellectual rigour. Christians generally don't do that. And it evolved adding different elements, like the Jesus story in the 4th Century, and I think Satan wasn't added until a century or two after that. I'd have to check that, but this would surprise Christians I'm sure. Now the Virgin Mary story came much later, and she's been turning up in French toast and alike ever since. Also let's not forget that at the the time of the canonisation of Jesus, 4th Century, there were all these different Christian groups with various versions of who and what Jesus was. Well they were hunted down and dealt with by the Christians. Bart Erhman's last book, How Jesus Became God, goes into that in detail. Take any point in Christian history. It's not a nice story. And it still isn't.

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.