Trinity web sites for Muslims:
http://www.the-good-way.com/eng/article/a06.htm#2.00
http://www.arabicbible.com/islam/christian_doctrine.htm
Following from: http://www.the-good-way.com/eng/article/a06.htm#2.1
THE QUR'ANIC TESTIMONY OF THE CHRISTIAN HOLY TRINITY:
To our dear Muslim Brothers:
Perhaps you are amazed, my dear friend, that the Qur'an mentions the Trinity of the one God exactly as Christians believe in it. We have already seen that the Trinity of Christianity is the nature of God. His Word, and His Spirit. This is the same Trinity that the Qur'an mentions, "But Jesus Christ, son of Mary is the messenger of God and His word and spirit of Him that He gave to Mary." (Sura al-Nisa 4:171) In this verse it is clear that God has :
a personality - "messenger of God"
a word - "and His word"
a spirit - "and a spirit from Him"
This testimony of the Qur'an for the creed of the Trinity is what we Christians proclaim and no more. It does not proclaim polytheism, but rather it proclaims that there is no God but Him.