Three 33% workers going on strike has the same effect as one full time worker going on strike.
Are you sure? If 3 full-timers call a 3 day strike, the company loses 9 man days. If it's 3 33% workers who are each rostered for one day the company only loses 3 man days.
Therefore the original point stands - i.e. that, although part-timers boost a strike vote out of proportion to their working hours (unfairly according to some), when it comes to actually striking their one vote is not as powerful as that of a full-timer.