Put simply, they don't believe in it, not the way mainstream Christianitydoes. They believe in all three entities, but that all three entities are distinct, separate beings.

THey refer to the three as "The Godhead". There's more here: http://www.lds.org/topics/godhead?lang=eng

The true doctrine of the Godhead was lost in the apostasy that followed the Savior's mortal ministry and the deaths of His Apostles. This doctrine began to be restored when 14-year-old Joseph Smith received his First Vision (see Joseph Smith—History 1:17). From the Prophet's account of the First Vision and from his other teachings, we know that the members of the Godhead are three separate beings. The Father and the Son have tangible bodies of flesh and bones, and the Holy Ghost is a personage of spirit (see D&C 130:22).

Put simply, they don't believe in it, not the way mainstream Christianity does. They believe in all three entities, but that all three entities are distinct, separate beings.

They refer to the three as "The Godhead". There's more here: http://www.lds.org/topics/godhead?lang=eng

The true doctrine of the Godhead was lost in the apostasy that followed the Savior's mortal ministry and the deaths of His Apostles. This doctrine began to be restored when 14-year-old Joseph Smith received his First Vision (see Joseph Smith—History 1:17). From the Prophet's account of the First Vision and from his other teachings, we know that the members of the Godhead are three separate beings. The Father and the Son have tangible bodies of flesh and bones, and the Holy Ghost is a personage of spirit (see D&C 130:22).