Muslims do not believe in Trinity because Quran says whosoever ascribes partners with Allah will be forbidden Paradise and that Jesus is not God.Sura 5:72 They surely disbelieve who say : Lo! Allah is the MESSIAH, son of Mary. The MESSIAH (himself) said : O Children of Israel, worship Allah, my Lord and your Lord. Lo! whoso ascribeth partners unto Allah, for him Allah hath forbidden Paradise. His abode is the Fire. For evildoers there will be no helpers.
Did the spirit who reveal to prophet of Islam say "I am Gabriel"? Then who say that unidentified spirit is Gabriel? A Christian who never met the spirit said so. That was an invalid witness. Gabriel always make himself known unlike that unidentified spirit.
Dan 8:16And I heard a man's voice between the banks of Ulai, which called, and said, Gabriel, make this man to understand the vision.
Luk 1:19 And the angel answering said unto him, I am Gabriel, that stand in the presence of God; and am sent to speak unto thee, and to show thee these glad tidings.
1) Father is God and not God is the Father
Father in heaven is God because God resides in heavenly throne.
2) Jesus Christ is God and not God is Jesus Christ.
2.1 Devil knew Jesus Christ title as "the Son of God"
Mat 4:3 And when the tempter (devil) came to him, he said, If thou be the Son of God, command that these stones be made bread.
2.2 Demons knew Jesus as the Son of God.
Mat 8:29And, behold, they cried out, saying, What have we to do with thee, Jesus, thou Son of God? art thou come hither to torment us before the time?
2.3 Men knew Jesus as the Son of God
Mat 14:33Then they that were in the ship came and worshiped him, saying, Of a truth thou art the Son of God.
Mat 26:63But Jesus held his peace. And the high priest answered and said unto him, I adjure thee by the living God, that thou tell us whether thou be the Christ, the Son of God.
Mat 26:64 Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man sitting on the right hand of power, and coming in the clouds of heaven.
Joh 10:36 (Jesus said)Say ye of him, whom the Father hath sanctified, and sent into the world, Thou blasphemest; because I said, "I am the Son of God"?
3) Holy Spirit is God and not God is Holy Spirit.
Blasphemy against Holy Spirit is an eternal sin. Only sin against an infinite being is eternal. So Holy Spirit is God. But to say God is Holy Spirit is false statement because the order cannot be reversed.
Mat 12:31 (Jesus said) Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven unto men.
Holy Spirit is God because He came from the Father and has personality as He shall teach and help to remember all things
Joh 14:26 But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.
Trinity is Three in One; the Father, the Son Jesus and Holy Spirit are One.
Mat 28:19Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost
1Jo 5:7For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
1Jo 5:8 And there are three that bear witness in earth, the spirit, and the water, and the blood: and these three agree in one.
God Himself has testified through Jesus Christ and then confirmed by the Holy Spirit.