No it not pagan ... because Islam says so?
The word "Trinity" only appears in the Qur'an in verses [Qur'an 4:171] and [Qur'an 5:73]:
O People of the Book, commit no excesses in your religion; nor say of God anything but the truth. The Messiah Jesus son of Mary was (no more than) a Messenger of God, and His Word, which He bestowed on Mary, and a Spirit proceeding from Him; so believe in God and His Messengers. Say not "Trinity": desist! It will be better for you: for God is One: Glory be to Him! (far exalted is He) above having a son. To Him belong all things in the heavens and on earth. And enough is God as a Disposer of affairs. (Qur'an [Qur'an 4:171])
They do blaspheme who say: God is one of three in a Trinity: for there is no god except One God. If they desist not from their word (of blasphemy), verily a grievous penalty will befall the blasphemers among them. (Qur'an [Qur'an 5:73])
http://en.wikipedia.org/wiki/Trinity_in_Islam