In conferring the rite of Baptist, Joseph Smith writes in the Doctrines and Covenants as follows:
73 The person who is called of God and has authority from Jesus Christ to baptize, shall go down into the water with the person who has presented himself or herself for baptism, and shall say, calling him or her by name: Having been commissioned of Jesus Christ, I baptize you in the name of the Father, and of the Son, and of the Holy Ghost. Amen.
And yet, they are also explicitly non-Trinitarian:
Latter-day Saints believe that the simplest reading of the New Testament text produces the simplest conclusion — that the Father, the Son and the Holy Ghost are separate and distinct personages, that They are one in purpose. We feel that the sheer preponderance of references in the Bible would lead an uninformed reader to the understanding that God the Father, Jesus Christ and the Holy Ghost are separate beings. That is, one must look to the third- and fourth-century Christian church, not to the New Testament itself, to make a strong case for the Trinity
So, if they are tri-theistic rather than Trinitarian, why would one baptise in three personages? My understanding is that Baptism is supposed to be an identification with one person, so having "three" names doesn't seem to gel. What is the justification here?