Source : Webster's Revised Unabridged Dictionary (1913)

Triple \Tri"ple\, v. i. [imp. & p. p. {Tripled}; p. pr. & vb. n.
   {Tripling}.] [Cf. F. tripler. See {Triple}, a.]
   To make threefold, or thrice as much or as many; to treble;
   as, to triple the tax on coffee.

Source : WordNet®

tripling
     n : increase by a factor of three