trichotomous
adjective
tri·chot·o·mous trī-ˈkä-tə-məs 
: divided or dividing into three parts or into threes
trichotomous branching
trichotomously adverb
Word History
Etymology
Late Greek trichotomein to trisect, from Greek tricha in three (akin to treis three) + -tomein (akin to temnein to cut) — more at three, tome
First Known Use
1800, in the meaning defined above