Isn't the Trinity three gods?
No. Christians confess one God, as the Shema says. The Father, the Son and the Holy Spirit are one God, not three gods, and never God, Jesus and Mary.
Jesus quoted the Shema as the first commandment. He also commanded baptism in the one name of the Father and of the Son and of the Holy Spirit. The Quran's picture of a Trinity including Mary, or of God as the third of three, is a Trinity Christians also reject.
29Jesus answered, “The greatest is: ‘Hear, Israel, the Lord our God, the Lord is one.
Mark 12:29 (WEB) · context
19Go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,
Matthew 28:19 (WEB) · context
73They surely disbelieve who say: Lo! Allah is the third of three; when there is no Allah save the One Allah. If they desist not from so saying a painful doom will fall on those of them who disbelieve.
Quran 5:73 (Pickthall) · context
116And when Allah saith: O Jesus, son of Mary! Didst thou say unto mankind: Take me and my mother for two gods beside Allah? he saith: Be glorified! It was not mine to utter that to which I had no right. If I used to say it, then Thou knewest it. Thou knowest what is in my mind, and I know not what is in Thy Mind. Lo! Thou, only Thou, art the Knower of Things Hidden?
Quran 5:116 (Pickthall) · context