Source : Webster's Revised Unabridged Dictionary (1913)

Three-ply \Three"-ply`\, a.
   Consisting of three distinct webs inwrought together in
   weaving, as cloth or carpeting; having three strands;
   threefold.

Source : WordNet®

three-ply
     adj : having a thickness made up of three layers or strands;
           "three-ply cloth"; "three-ply yarn"