Thank you. Christian doctrine is that the Trinity is the combined three aspects of God. These are: the Father, the Son, and the Holy Spirit. God has three aspects and one unified essence. In Christian belief, there is only one God, and he has three aspects. There is a fundamental unity of
the three aspects in the one God. Hence, Christianity is monotheistic.
The Trinity is revealed in the Bible. The existence of the Trinity is simply not up for negotiation or compromise by Christians. Some offshoots of Christianity like the Jehovah's Witnesses deny the Trinity (usually they say they deny the divinity of Jesus Christ), but it is questionable whether those offshots are truly Christian at all.
In contrast, the Jewish holy book (the Tanakh) never explicitly endorses the Trinity, but Judaism does not exlicitly deny the Trinity either. Furthermore, Judaism does accept a second, "female aspect" of God (of a name I forget). Clearly there is tension between Christianity and Judaism, but that tension is theoretically reconcilable. According to the Koran, the Trinity is not God. Period.
In his piece, Taheri is speaking to Muslims, and perhaps feels pressure to give the orthodox Muslim doctrine that Christianity is "polytheistic." There is no excuse for such an error, however. That Islam is in grave error on the subject does not excuse anyone.
In Christian belief, to deny the Trinity is to deny God. It is as simple as that.
I realize people like Taheri may disagree, but that does not mean I must respect error-ridden opinions.
I'm not sure it's warranted to conflate Judaism with the Tanakh, especially from a Christian perspective. After all, if you're a Christian, you must believe the Old Testament points to the New. And even not from a Christian perspective, there's lots of more or less official Jewish writing (the Talmud, for example) outside the Old Testament. If I'm not mistaken, these do deny the Trinity, in reaction against Christianity.
As for the Trinity in the Old Testament, you should look up the Angel of the Lord. The "female aspect" idea comes from Wisdom in the Proverbs, but it makes a lot more sense to consider Wisdom a literary personification of "regular" wisdom rather than a person. Although if I'm not mistaken, Wisdom shows up in the Kabbalah as a person or emanation or something, and there's also Gnostic weirdness associated with "her" (Gnostic weirdness is a redundancy).
You have outlined the orthodox Christian position in a nutshell.
Personally I am quite comfortable with the notion of a triune God. I can't see any way that position can be turned into polytheism or anything else at all but monotheism without distortion to the point of lying.
Of course, that has never been out of bounds to people with an axe to grind, as seems to be the case with Muslims.
Frankly, I think it stems from the early days of the religion when it was truly a cult and were trying to find ways to set themselves apart from Christianity and Judaism, from where it clearly derives.