A 120 triple should indeed be smooth - but it needs the balancer shaft to sort out the end-to-end vibrations which exist.
See:
http://autozine.kyul.net/technical_s...1.htm#Inline-3
I'm NOT an engineer and am happy to be be advised to the contrary, but this article makes sense to me.
NB: The Triumph motors have a 123 firing order rather than the 132 illustrated, but I don't think this makes any difference.
