The Collaborative International Dictionary of English v.0.48:

Trimorphic \Tri*mor"phic\, Trimorphous \Tri*mor"phous\, a. [Gr.
   ? three-formed; ? (see Tri-) + morfh` form.]
   Of, pertaining to, or characterized by, trimorphism; --
   contrasted with monomorphic, dimorphic, and
   polymorphic.
   [1913 Webster]