IS ALLAH (S.W) ONE OR THREE?

Monotheism, which is what Mohammad (PBUH) carried to humanity, is the faith and belief of all the prophets (PBUT) before him. [Not an apostle did We send before thee without this inspiration sent by Us to him: that there is no God but I; therefore worship and serve Me.] (Holy Quran, 21: 25) However, Christians believe contrary to the Muslims, as they believe that Jesus (PBUH) is the Son of Allah (S.W), and some of them believe that he is Allah (S.W) himself. They also believe that He, (Allah (S.W)) came down from heaven, incarnated, was slapped, suffered and crucified, for the atonement of the sin of humanity, which they inherited from their father Adam. 

I question, where did they derive this belief, and is there some evidence in their books that support it? [ Say," Bring your convincing proof.] (Holy Quran, 21: 24)