They are One God indeed. Trinity seems to be the best argument agaist itself.
For they say there is One God (Father, Son, and Spirit)
The Father is God (Father, Son, and Spirit)
The Son is God (Father, Son, and Spirit)
The Holy Ghost is God (Father, Son, and Spirit).
So there is truly One God. And He is Father, SOn, and SPirit.
To suggest that they are One in unity is to suggest that there is more than one united. And that is polytheism regardless of their claims that it is one GOd. Becuase to them, that One (monotheism) God is not one (numerically) but three (plural/polytheistic) in unity as one.
Love Always,