Monotheism is the belief in One God. 

Deuteronomy 6:4

“Hear, O Israel! The Lord is our God, the Lord is one!

Does Christianity introduce the concept three Gods?

Christianity is monotheistic. 

1 Corinthians 8:4-8

Therefore concerning the eating of things sacrificed to idols, we know that there is no such thing as an idol in the world, and that there is no God but one. For even if there are so-called gods whether in heaven or on earth, as indeed there are many gods and many lords, yet for us there is but one God, the Father, from whom are all things and we exist for Him; and one Lord, Jesus Christ, by whom are all things, and we exist through Him. However not all men have this knowledge; but some, being accustomed to the idol until now, eat food as if it were sacrificed to an idol; and their conscience being weak is defiled. But food will not commend us to God; we are neither the worse if we do not eat, nor the better if we do eat.