Why do Christians believe in three gods?

It is true that the Qur'an condemns who believe: "... Allah is the third of three ... " (Surah 5:73, tr. Pickthall). Elsewhere, we are told, "And behold! Allah will say: "O Jesus the son of Mary! Didst thou say unto men, 'Worship me and my mother as gods in derogation of Allah?" (Surah 5:116, tr. Yusaf Ali)

What the Qur’an is condemning is a trinity made of God (Allah), Mary and Jesus.  However, Christians do NOT believe in three gods as the Bible says: "The Lord our God, the Lord is One" (Deuteronomy 6:4).

It is recorded in the gospel of Mark that Jesus used this very verse himself when asked, "Of all the commandments, which is the most important?": "The most important one," answered Jesus, "is this: 'Hear, O Israel, the Lord our God, the Lord is one. Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.'" (Mark 12:29,30)

Indeed, we believe in ONE God and love him with all our heart, soul, mind and strength. While our Muslim friends hold the idea of Tawhid, we as followers of Christ hold to God's unity in Trinity and thus one declares, I believe in one God who has revealed himself in the Scripture as the Father, the Son and the Holy Spirit…..

We only know as much as what Jesus has revealed about God. That Great Day in His presence we will know more about HIM.