string would be tough at that scale but weirdly might be easier to make that measurement with than a ruler. just cut your string to 9.5mm length, then divide it by three, one of those divisions is your target length and you even have the other two to check your division's margin of error against.

to calculate and measure with a ruler would have you measuring 3.16666... which i would not be able to measure with a ruler. now a vernier caliper would be the right tool to make that measurement, and even if mine only had tenths of a millimeter id just round to 3.15mm and mark in between the mm graduations when forced to use that group of tools.

of course that's if you know how to use a set of calipers. its not as easy as one might think.

lets not forget that your response to the one third of 9.5 conundrum, which was posed by a metric defender, is:

okay then my answer to the hypothetical is 9.5/3, which is every bit as easy to find on any measurement device, or to use for any practical purpose, as 1/24th

which is literally not true as i have explained about the 24th scale ruler and even my digital calipers don't do repeating digits or express portions of metric measurements in fractions.

of course, in a real world situation i'd never be trying to mark 3.1666...mms because it's 1/3 of a thou under 1/8 inch. i'd just mark an eighth of an inch like a normal person.

even using a calculator to figure out the length suggests that a person in that conundrum stop using the metric side of their ruler.