triune

from WordNet (r) 3.0 (2006)

triune
    adj 1: being three in one; used especially of the Christian
           Trinity; "a triune God"

from The Collaborative International Dictionary of English v.0.48

Triune \Tri"une\, a. [Pref. tri- + L. unus one. See {One}.]
   Being three in one; -- an epithet used to express the unity
   of a trinity of persons in the Godhead.
   [1913 Webster]