If all the toons are under the same account and you want to use my UI for 2 out of 3 of them , you just dissable all the addons for the 1 account that you dont want my UI to run on, that is if that 1 toon uses the default blizzard UI. If that one account uses a custom UI you would need some sort of manager to do that, I believe wowace.com has one, though I have never used it and can't recall what it is called.
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