Source : Webster's Revised Unabridged Dictionary (1913)

Three-cornered \Three"-cor`nered\, a.
   1. Having three corners, or angles; as, a three-cornered hat.

   2. (Bot.) Having three prominent longitudinal angles; as, a
      three-cornered stem.

Source : WordNet®

three-cornered
     adj 1: having three corners; "a three-cornered hat"
     2: involving a group or set of three; "a three-cornered race"