The simple answer is that they all believe in one god but the more complex answer is in the way they started. Christianity grew out of Judaism. As you may know Jesus was a Jew and only later did Christianity take the form we see today.

It is clear that also the Islam, that came later, was influenced  by these two religions. there are many motifs in the Koran that seem, to the eyes of the scholar, as branches that grew from the tree of Judaism an Christianity.

We can also see through history many conflicts between believers of these religions. These conflicts made history what it is today.

To end the discussion I will just add that according to Judaism there is a question whether Christianity is a real monotheistic religion The trinity speaks of three parts to god and that is in contradiction to the "one god" paradigm.

Hope this was helpful. 

Hope I was helpful.