Training None Intent in LUIS

None intent is not made for "Negative variations" but to tag everything that is not managed by your other intents.

So you should add utterances corresponding to cases that your bot cannot handle but linked to your context (your 3rd idea).

For example in one of my projects, None intent is trained with other use-cases of my customer than the one I will treat with my bot, based on my customer logs. And it's helping avoiding to throw an intent in a bad case