The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tripling
    n 1: increase by a factor of three