Oh, Im finally starting to get it! Youve just got to word it correctly! You can say, So the Father is God, the Son is God, and the Holy Spirit is God. And youre monotheistic provided you add, And yet they are not three Gods, but one God. Counting to three really isn't what it's all about, after all. It's the disclaimer that makes it all okay!
For your information, heres what The Book of Mormon has to say:
And now, behold, my beloved brethren, this is the way; and there is none other way nor name given under heaven whereby man can be saved in the kingdom of God. And now, behold, this is the doctrine of Christ, and the only and true doctrine of the Father, and of the Son, and of the Holy Ghost, which is one God, without end. (2 Nephi 31:21)
Now, this restoration shall come to all, both old and young, both bond and free, both male and female, both the wicked and the righteous; and even there shall not so much as a hair of their heads be lost; but every thing shall be restored to its perfect frame, as it is now, or in the body, and shall be brought and be arraigned before the bar of Christ the Son, and God the Father, and the Holy Spirit, which is one Eternal God, to be judged according to their works, whether they be good or whether they be evil. (Alma 11:44)
And after this manner shall ye baptize in my name; for behold, verily I say unto you, that the Father, and the Son, and the Holy Ghost are one; and I am in the Father, and the Father in me, and the Father and I are one. (3 Nephi 11:27)
And he hath brought to pass the redemption of the world, whereby he that is found guiltless before him at the judgment day hath it given unto him to dwell in the presence of God in his kingdom, to sing ceaseless praises with the choirs above, unto the Father, and unto the Son, and unto the Holy Ghost, which are one God, in a state of happiness which hath no end. (Mormon 7:7)
So, apparently there really isnt a Christian denomination that is not monotheistic. At least not mine!