Now, my friend, I come to the question you have been wanting to ask ever since I told you that Christians believe in One God. For I know you have heard that Christians worship three gods: God and Jesus and His mother Mary, and that these three are called the "Trinity". As I explained before, if Christians have worshipped any of God's creatures in place of, or along with, the Creator, they have greatly erred. The Virgin Mary is a very holy person, but she should never be given the worship which belongs to God only. It is true that Christians worship Jesus Christ, but we do this not because He was a holy prophet, but because He was from eternity the Word of God and the Son of God, perfectly one with God. He is not a man whom we have exalted to be a god along with the One True God. Rather, He is from eternity one with God, and He became a man to save sinful men and bring them back to God. Hence He is worthy of worship, for He is God indeed.

Also, we have just explained that the Holy Spirit is not one of God's creatures, but is one with God and is God. The Spirit does in and for man what only God is able to do, and is therefore worthy of worship just as the Son is worthy of worship. What then are we to say? Are there three Gods, Father, Son and Spirit. The Father, whom human eye has never seen, is the source of spring of Deity. Once in human history the Son perfectly manifested the Unseen Father. The Spirit was given by the Father and the Son to accomplish the divine purpose in men.

Read full article here: WHAT IS MEANT BY THE TRINITY? - An Explanation of the Trinity for Muslims