You're confusing the distinctives being and person. A being is what makes something what it is. It is the essence or nature of that thing. A person, in the theological sense that I am using it, is the ability to use personal attributes. (To feel, speak, have a will, etc.) Christians, Jews, and Muslims are all Monotheistic. That is, we all believe in one being of God. That is what determines Monotheism or Polytheism: The number of beings of God. The difference that arises between Jews and Muslims and Chrsitians is that the two former groups are Unitarian Monotheists and the latter is Trinitarian Monotheists. Are disagreement is on the number of persons whithin the one being. Mormons, on the other hand, readily affirm multile beings of God; hence the Polytheistic classification.