The Collaborative International Dictionary of English v.0.48:

Ternion \Ter"ni*on\, n. [L. ternio, fr. terni three each. See
   Tern, a.]
   The number three; three things together; a ternary. --Bp.
   Hall.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ternion
    n 1: the cardinal number that is the sum of one and one and one
         [syn: three, 3, III, trio, threesome, tierce,
         leash, troika, triad, trine, trinity, ternary,
         ternion, triplet, tercet, terzetto, trey, deuce-
         ace]