The Collaborative International Dictionary of English v.0.48:

Trisulc \Tri"sulc\ (tr[imac]"s[u^]lk), n. [L. trisulcus; tri-
   (see Tri-) + sulcus a furrow.]
   Something having three forks or prongs, as a trident. [Obs.]
   "Jupiter's trisulc." --Sir T. Browne.
   [1913 Webster]