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]WordNet (r) 3.0 (2006):
triune adj 1: being three in one; used especially of the Christian Trinity; "a triune God"