The claim is made that the concept of the Triune Godhead was first introduced by Paganism (Nimrod, Semiramis, Tammuz), and it was through these pagan religions that Satan began to confuse people with the erroneous idea of three co-equal beings who should be worshiped as one God.
While it is true that a trinity of gods was worshiped in some pagan religions, the question is whether Christianity got this concept from these pagan religions or whether the pagan concept is a counterfeit of the truth of the triune God.
Just because a trinity of gods existed in paganism before the rise of Christianity does not prove that the Christian doctrine came from paganism. It was believed by the pagans that Tammuz died and was resurrected (see SDA Bible Dictionary, pg 1064; Hislop, The Two Babylons, https://bit.ly/3sUtbsd). This was celebrated every year in their worship. Does this make the Christian doctrine of the death and resurrection of Jesus pagan? Of course not.
The virgin birth was likewise to be found in Paganism. The miraculous birth of Tammuz was claimed to be a "virgin birth" (Justin Martyr, Ante-Nicene Fathers, Vol 1, https://bit.ly/3DAEYAH). Does the fact that the Pagans believed in a virgin birth make the virgin birth of Jesus a pagan doctrine? No, because it is clearly founded on the Bible. As Justin Martyr rightly observed, "I understand that the deceiving serpent counterfeited also this."
Satan's primary mode of operation in his work of deception is to counterfeit the things of God. Thus we find that Satan has a counterfeit "creation" (evolution), a counterfeit of God's ideal for marriage, a counterfeit religious system, counterfeit "gospel", counterfeit miracles, counterfeit revival, counterfeit prophets, a counterfeit Sabbath, and the crowning act of his deception to enforce the counterfeit Sabbath: a counterfeit Christ. Wherever there is the false there must be the true. Satan never counterfeits what does not exist. Thus it should not be of any surprise that Satan would have a counterfeit trinity in the closing days of Earth's history, the prototype of which is the pagan trinity of false gods.
A hierarchical view of the Godhead came into the Christian church with Constantine. Prior to this the early church community was modeled after the divine model of co-equality and mutual submission. But after the conversion of Constantine the church structure reflected the hierarchical, vertical model that was prevalent in Roman society. The patron-client relationship in Roman society involved submission that was bottom-up, where everyone from slave to aristocrat was bound to display respect to someone more powerful than himself, up to the emperor. This became the view of the Godhead as well. So the heavenly hierarchy was such that the Father was on top, followed by the Son, and then the Spirit, followed by angels, then the Virgin Mary, the saints, and so on. If you wanted to approach God you had to have a patron saint or Mary.
The view of God the Father being the "One True God" to the exclusion of Christ or the Spirit, or that only the Father is God in personality, is a trend towards a hierarchical view of the Godhead where there are degrees of divinity and one member of the Godhead is "more God" than another member. In pagan religions there are concepts like chief deities, minor deities, intermediary deities, and pantheons and hierarchies. The papal hierarchical view of the Godhead is not only unscriptural but is quite pagan, and the views of the so-called "One True God" movement are mirroring a papal/pagan view of the Godhead.
In the book of Revelation we find Satan's end-time strategy outlined in detail. A significant aspect of this strategy is to counterfeit the work of the triune God in preparing a people ready for His return. We find that there are three entities that comprise this false trinity: the Dragon (Satan - Rev 12:9), the Sea Beast (Religio-Political System), and Land beast (False Prophet - Rev 19:19,20). The activity of this counterfeit trinity is all about defeating God's remnant people through whom Christ will crush the serpent (see Gen 3:15; Rom 16:20; Rev 12:17).
In the Genesis account of creation we see God the Father, Christ the Son, and the Holy Spirit all involved in creation. Each member of the Godhead contributed to the creation of man ("Let us create man"), and man was made in God's image ("in our image"). In Genesis 2:7 God formed Adam out of the dust of the earth and breathed into his nostrils the breath of life. But in Revelation 13 we find a counterfeit "creation" in which the Land Beast breathes life into the image of the beast (verse 15, lit. "to give the breath of life"). Thus we find that Satan's end-time strategy involves a counterfeit trinity that gives life to the image of the beast that had a deadly wound (a counterfeit of Christ's death and resurrection).
The evidence from Scripture is that Satan has produced a counterfeit of the truth of the triune God. Those who deny this truth are themselves playing into the hands of the enemy by ascribing to evil agencies what belongs to God.