You could do that. In practice you may need less extra space than that because the chainstays taper outward slightly and the granny chainring has more room. Regardless it's not necessarily a bad idea to try on 5mm longer as a first guess.

Also, minor differences in BB spindle taper will change how far the crank sticks out. ISO and JIS differences aren't quite as cut and dried as you might think, but even so, stick with ISO. It may actually be easier to fit a new BB than try and find an NOS campy spindle in the correct length for a triple.

You may actually want to have an LBS do this, because sometimes it takes a couple try and fits to get the right size.