The Collaborative International Dictionary of English v.0.48:

Trivalent \Triv"a*lent\, a. [Pref. tri- + L. valens, -entis, p.
   pr. See Valence.] (Chem.)
   Having a valence of three; capable of being combined with,
   substituted for, or compared with, three atoms of hydrogen;
   -- said of triad atoms or radicals; thus, nitrogen is
   trivalent in ammonia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

trivalent
    adj 1: having a valence of three