In #2, eliminate "in". In #3, replace "in" with "where".

Yes

Yes, they are okay. I agree with Dan, however, that the second one is a bit awkward. I would prefer "You're meddling when you shouldn't."

The basically all mean the same thing. To me the second sentence sounds incorrect. If you say 'meddling in' there needs to be a direct object, So either: You're meddling. OR You're meddling in other peoples' business. etc.

Yes, all of these work.