The Collaborative International Dictionary of English v.0.48:

Trilogy \Tril"o*gy\, n. [Gr. trilogi`a; pref. tri- (see Tri-)
   + lo`gos speech, discourse: cf. F. trilogie.]
   A series of three dramas which, although each of them is in
   one sense complete, have a close mutual relation, and form
   one historical and poetical picture. Shakespeare's " Henry
   VI." is an example.
   [1913 Webster]

         On the Greek stage, a drama, or acted story, consisted
         in reality of three dramas, called together a trilogy,
         and performed consecutively in the course of one day.
                                                  --Coleridge.
   [1913 Webster] Triluminar

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "trilogy":
   clover, deuce-ace, leash, set of three, shamrock, tercet, ternary,
   ternion, terzetto, three, threesome, tierce, trefoil, trey, triad,
   trialogue, triangle, tricorn, trident, triennium, trihedron,
   trimester, trine, trinity, trinomial, trio, triphthong,
   triple crown, triple threat, triplet, triplopy, tripod, triptych,
   trireme, triseme, triskelion, trisul, triumvirate, triunity,
   trivet, troika