Wordnet 3.0
NOUN (1)
1. 
 (Christianity) the heretical belief that the Father and the Son and the Holy Spirit are three separate gods; 
The Collaborative International Dictionary of English v.0.48:
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.
   [1913 Webster]
WordNet (r) 3.0 (2006):
tritheism
    n 1: (Christianity) the heretical belief that the Father and the
         Son and the Holy Spirit are three separate gods