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

WordNet (r) 3.0 (2006):

trilogy
    n 1: a set of three literary or dramatic works related in
         subject or theme

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



The Free On-line Dictionary of Computing (30 December 2018):

Trilogy

    A strongly typed logic programming language
   with numerical constraint-solving over the natural
   numbers, developed by Paul Voda 
   at UBC in 1988.  Trilogy is syntactically a blend of
   Prolog, Lisp, and Pascal.  It contains three types of
   clauses: predicates (backtracking but no assignable
   variables), procedures (if-then-else but no backtracking;
   assignable variables), and subroutines (like procedures, but
   with input and system calls; callable only from top level or
   from other subroutines).

   Development of Trilogy I stopped in 1991.  Trilogy II,
   developed by Paul Voda 1988-92, was a declarative general
   purpose programming language, used for teaching and to write
   CL.

   (http://fmph.uniba.sk/~voda).

   ["The Constraint Language Trilogy: Semantics and
   Computations", P. Voda, Complete Logic Systems, 741 Blueridge
   Ave, North Vancouver BC, V7R 2J5].

   (2000-04-08)