Good question there...
I have no idea why one must say 'Stop' three times when once would probably suffice to say nothing of saying it twice. That is the way it was presented to me as a Company SOP, so that's the way we do it.
It might go way back to something that happened in the FAA or the RAF. Something like that, I suppose that is now long gone and forgotten as the reason for this arcane practice. It isn't worth worrying about, so that I save my energy for questions about other practices that present bigger potential problems.
I did hear one about a co-pilot who looked down in the dumps, when the Captain said, 'Cheer up!' You know what happened next...
We have a lot of different cultures working together so that whatever the call or action we need to stick to the agreed-upon script. Here I can get some ideas, at least, about possible changes to the script.