prove that :-

tan3x tan 2x tan x = tan 3x - tan 2x - tan x

tan3x = tan(2x +x)

tan3x   = (tan 2x + tanx )/  1- tan2x tanx  [as tan(A +B )= tanA + tanB / 1- tanA tanB ]

by crossmultiplying

  tan3x - tan3x tan2x tanx = tan2x +  tanx

  hence tan3x - tan2x -tanx = tan3x tan2x tanx