%triatomic%

a. [ Pref. tri- + atomic. ] (Chem.) (a) Having three atoms; -- said of certain elements or radicals. (b) Having a valence of three; trivalent; sometimes, in a specific sense, having three hydroxyl groups, whether acid or basic; thus, glycerin, glyceric acid, and tartronic acid are each triatomic. [ 1913 Webster ]