Source : Webster's Revised Unabridged Dictionary (1913)

Tritheism \Tri"the*ism\, n. [Pref. tri- + Gr. ? God: cf. F.
   trith['e]isme.]
   The opinion or doctrine that the Father, Son, and Holy Spirit
   are three distinct Gods.

Source : WordNet®

tritheism
     n : (Christianity) the heretical belief that the Father and the
         Son and the Holy Spirit are three separate gods