Floating point numbers can accurately represent integers, so if you have all your prices in cents, you end up with (x * 3) / 3, where each number in that calculation is an integer. No inaccuracy there. Of course there is no reason for a register to actually perform this division in a three-for-one action, it can just replace (x * 3) with x, or subtract (x * 2).

I agree as much as the next person that you shouldn't represent money with floating point numbers, but I disagree that cash register software written in Javascript must automatically be incorrect (or more so that similar software in a different language). And I don't even like Javascript.