The Mormon understands the Christian idea of Trinity and rejects it. The Mormon believes that he worships the same God as we, but that he has more information about Him and therefor worships him more intelligently in such knowledge.
So rather than try to convince them of the truth of the Trinity, instead use it to demonstrate that we do not worship the same God in a different way, but that we worship different Gods. Then they have to make a decision about which God is God.
My God the Father is the only God that ever was or will be. He said “Before me there was no God formed, neither shall there be after me.” So I do not worship a God who is one in a long succession of gods. He is spirit and is worshiped in spirit and truth. (and a spirit does not have flesh and bones).
My God the Son, was the same eternal God as God the Father, yet he emptied himself of deity in the incarnation in order to save men by the shedding of his blood on the cross. I do not worship a polytheistic God the Son who is a second God to God the Father and sweat in Gethsemane for some sins.
My God the Holy Spirit is the same God as the Eternal God the Father and God the Son, who is a person, and who reveals himself in the hearts of men. I do not worship a God the Holy Spirit who is an impersonal force, or who is a disembodied spirit looking for a body of flesh in the person of Joseph Smith.
The Father, Son and Holy Ghost are one God, which is incomprehensible to our finite minds, yet who has revealed himself so that we might know him. The Father knows things that the Son and Holy Ghost don’t. (The time of Christ’s second coming). The are and have been one since the beginning with the exception of the cross where God was torn and symbolized by the veil, the parting of the sea, the cleft rock, etc.
The Holy Gost is a separate person than the Son as symbolized by the descent of the dove upon Jesus at his baptism, and that Jesus would send him to us when He had gone away.
I worship a God who is so big he fills the universe and so small he fits in my heart.
It is not that I do not know what the Mormons teach, it is that I know it and actively reject it as a false God made in the image of men. We do not worship the same God in a different way since your god is just one of many gods and My God is the only true and real God of all the universe, not just of this planet. In fact the god of this world is Satan as taught in the Bible, and the teachings of the Mormon god, that men can become gods, are a teaching of the serpent from the garden, and I actively reject it.
Choose you this day whom you will serve. The God of the universe, or the small and capricious god of Joseph Smith’s inventive mind.