Please excuse the cut and paste from my prior post, but you may have missed the origianl. I haven't seen a better response to your question than this:
The word for the Christian deity, "God," is a collective noun, like "team." The dictionary lists "God" as a synonym for "Godhead," and regardless of whether they subscribe to the concept of the "Trinity" (which I do not), most Christians do recognize that the Father, Son and Holy Ghost are all part of what the scriptures refer to as a "Godhead." So, in the sense that God=Godhead, it is accurate to say there is one "God" (meaning one Godhead comprised of three divine beings). Also, the three members of the Godhead share the title of "God." So we can accurately refer to any of them as "God." Just as we often say to a newlywed couple, "Now you two are one," we can say of the members of the Godhead, "These three are one." The mistake is in automatically assuming that they are physically "one," because they're not and there is nowhere in the Bible where we are told that they are.