The Collaborative International Dictionary of English v.0.48:

Trisect \Tri*sect"\, v. t. [imp. & p. p. Trisected; p. pr. &
   vb. n. Trisecting.] [Pref. tri- + L. sectus, p. p. of
   secare to cut. See Section.]
   1. To cut or divide into three parts.
      [1913 Webster]

   2. (Geom.) To cut or divide into three equal parts.
      [1913 Webster]