Obviously for this case we need to add a signifier for the countdown so it's clear to the other parties that you are aware of the standard and adhering to it before you even begin the countdown.

Like "ISO three two one GO!"

This is semi-backwards compatible, but still confusing for normies.

Even better, just make up new words where the ambiguity never existed. No numbers at all, just "glarp dook peow" and we always go on "peow" and always have. No backwards compatibility, but you'll be guaranteed that a person who doesn't understand will need clarification, and won't go unexpectedly through imagined agreement.

Or, if backwards compatibility is required, we could count up from 1 to 3... and our signifier phrase could be something like "awnthree". As a label for the standard we're using! Like, "awnthree, one, two, three".

I think that could work 😛