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.

So the second and third sentences wouldn't need the word "in." I will, however, add that the second and third feel a bit odd to my American English speaking ear. "You're meddling where you shouldn't" feels more natural to me (and can mean either "at a time when you shouldn't" or "in a place/situation where you shouldn't"). Similarly, but slightly different, the idiom "to stick one's nose" is almost always used with "where it doesn't belong"... "Where you shouldn't" isn't *technically* wrong, or even unintelligible, it's just something that would strike me as unusual.