See Alma 11:44. The Father, the Son, and the Holy Ghost are “one” not in physical being as some may claim, but rather in the sense of the dictionary definition “single through union, agreement, harmony . . . of a single kind, nature, or character, a state of unity, agreement, accord.” Each member of the Godhead is unique and separate, with his own important missions; and yet they are totally united in the purpose of bringing mankind to immortality and eternal life (see Moses 1:39).
The Savior himself spoke of this kind of oneness that characterizes the Godhead when he prayed to his Father that the Apostles “may be one, as we are” (see John 17:11, 21-22). He was not praying that the disciples become one in physical being, but rather that they might become completely unified in purpose. We too become one with God when, through faith and obedience, we become like them- when our thoughts and words and deeds are in total accord with those of the Father, the Son, and the Holy Ghost. See commentary on 3 Nephi 28:10.