Christians believe in one god just as jews do.
Trinity does not mean three gods.
Trinity means that god have had three forms, you know, like water -> ice, liquid, etc etc ...
The trinity is the father, the son and the holy spirit, for christians, the father is god as we all know him, the son is god when he came through jesus christ, the holy spirit is when god can move as spirit in each and every one of us. This doesn't contradict judaism, I don't know how that contradicts the image of god as the jews draws it ... its the same, except christians believed in jesus christ and that God have walked and talked with them through him (the son) and the holy spirit (the spirit of the man).