y=sinxsin2xsin3x
=-1/2(cos(x+2x)-cos(x-2x))sin3x
=-1/2sin3xcos3x+1/2sin3xcosx
=-1/4sin6x+1/4(sin(3x+x)+sin(3x-x)
=-1/4sin6x+1/4sin4x+1/4sin2x
y'=-1/4cos6x*6 +1/4cos4x*4+1/4cos2x*2
=-3/2cos6x+cos4x+1/2cos2x
y''=3/2sin6x*6-sin4x*4-1/2sin2x*2
=9sin6x-4sin4x-sin2x