The way I was taught is that the trinity is three separate beings. The Father (the person who is the father of Jesus and us) the Son (Jesus) and the Holy Ghost (the holy spirit). When it comes time for Judgment Day there was some debate among the Mormons on who would be sitting in the judgment seat, but most thought it would be the Father. Mormons believe that each of the three of the trinity has a purpose. God the father of all mankind. Jesus, the son of the Father, the medium through which we are forgiven of our sins. The holy ghost, the being who helps guide us in times of need.
Is Mormonism monotheistic? As a former Mormon, I would say I don't know, but mostly yes it is. The reason I say this is because Mormons believe that the Father is the only God they worship. They thank Jesus for dying on the cross, but he isn't the one they worship. However, they must go through him to get to the father. So perhaps it isn't. They close prayer in his name, but do not address the prayer to him. "Dear Heavenly Father, (say the prayer). In the name of Jesus Christ, Amen." is how I was taught to pray. So maybe it is...
I saw way too many conflicting teachings while I was in that religion, as well as most others I have explored. So I have chose to forsake all of them, and just believe in a higher power of some sort.
In the end, I think the lot of them are askew, but I am not one to try and debate or correct their beliefs. The family values of that church are quite well founded. It's just some of the teachings I had problems with.
As far as the higher power bit... I think the Intelligent Design group is way off too... That is a different debate though.