See this site for much more links and articles and Early Church Fathers also taught the Trinity.
Note: This is an interpretation of translators like Yusuf Ali who translates the term "Three" to be trinity. Other translations like Pickthall, Sale, simply translate the term literally as "Three"... ie. "Allah is not the third of Three." Yusuf Ali, therefore, reads into the text and interprets it as "trinity."
Muslims tend to see this the Trinity as a union between God and
Mary giving birth to Jesus.
The Christian will be puzzled
by such an allegation because the Christian doctrine of Trinity
has in the Godhead God the Father, God the Son (Jesus) and God
the Holy Spirit. Many researchers believed that either Muhammad
was wrong, or that it was refering to some heretical sects.
D.A. Rice writes in an internet newsgroup posting :
[...rest deleted...]I just want to remind readers of this thread to consider another possible interpretation, mentioned by Jamal-al-din Beaven (Jim Beaven) and myself. This is that there is evidence that an early sect of Christianity considered Mary as part of the Trinity. Please see his post and my post in this thread for more details.
In summary, parts of a document called the "Gospel of the Hebrews" has been quoted by the Church Fathers Origen and Jerome, writing in the 3rd and 4th centuries CE. This document explicitly identifies Mary with the Holy Spirit. This strongly suggests that the Christian sect which accepted this early Gospel believed that Mary was divine, and part of a Trinity.
It is possible that this sect still existed in the early 6th century CE, when the Qur'an was revealed. Therefore, it seems likely to me that Qur'an al-Ma'idah 5:119 (which says that Jesus (a.s.) will deny teaching that he and Mary were divine) is addressed specifically to this early Christian sect which probably considered Mary to be divine, and any other possible sects with a similar belief.
Contemporary Christians in general do not consider Mary to be divine. The Qur'an declares the error of the Trinity as believed in by many contemporary Christians in other verses.
The only text known to speak of Mary being the Holy Spirit is among the pseudepigrapha. In the Gospel of the Hebrews, we have :
"And the power came into the world and it was called Mary, and Christ was in her womb seven months...."Even so did my mother, the Holy Spirit, take me by one of my hairs and carry me away on to the great mountain Tabor."However, we should note that the mainstream of Christianity in Arabia in the 6th century A.D. is Nestorian, which denies that Mary is divine.