The pagan trinity was comprised of three Gods not one. The Greek triad of Zeus, Athena, and Apollo, the Hindu triad of Brahma, Vishnu, and Shiva the Egyptian triad of Isis, Horus, and Sub. They were all separate not united as the one God and almost unanimously had a mother involved as in a heavenly family. This was really tritheism, which has more in common with Mormonism than a triune God. Anti Trinitarians make usage of the statues with three heads and saying that is our pagan God.
Where did the pagans get a concept of three? Why not two or four? Where did they get the idea of a God in heaven anyway? What about their belief in a virgin and a son, where did that originate from? Romans 1:20-25 tells us that man from the beginning knew God."... and their foolish hearts were darkened" (verses 25) "they exchanged the truth for a lie, and worshipped and served the creature rather than the creator." When mankind fell into darkness of sin, they still retained some elements of the truth but distorted its meaning and it became lost.
As Walter Martin wrote "In order to find out if the doctrine of the Trinity is true, we do not look to see if it resembles paganism, but to the bible, to see if God teaches it in his word. Pagans also believe in the concept of God. does this mean that God must not be true? Pagans sleep. Does that mean sleeping is wrong? We must not dismiss an idea merely because it is held in common with those whom we may not approve." (the New Cults p.49)
Robert watts in New Apologetic says "The Pagan triads are "residuary fragments of their lost knowledge of God, not different stages in a process of evolution, but evidence of a moral and spiritual degradation." (Augustus H. Strongs systematic Theology p.352)
If we are to reject the concept of the triune God, a unified one, because of the pagan distortions, then we must reject much more than this. The pagan cultures also had a virgin birth and some even had a resurrection, Tammuz died and raised 40 days later. The pagan religions had a priesthood and sacrifices, this too became distorted with human sacrifices for Gods blessings. Some Pagans believed that certain gods became men. They had Biblical symbology such as a dove, the lamb, altars, their are many religions that have distortions of the Bibles account of the flood. Are we now to reject Genesis because of their misrepresentations. They also practiced tongues- ecstatic babble. All of these are counterfeits and distortions of truth, yet we are told since they have a counterfeit Trinity, we are to reject the Biblical Trinity on the same grounds. If you are going to do that, then you must reject all of it on the basis of it being found in some form in ancient paganism. You then annihilate the doctrine of Christ as the God/man, virgin birth, his sacrifice etc. All these are the proof of what Paul explained of what happened in history in Romans, that mankind had a true knowledge but refused to worship him and sank into idolatry.