They are all pretty strong, direct statements of feeling (or command) and would only be used fairly rarely (e.g. to a child/ pet who was misbehaving). "Stop it" is fairly neutral - although it does express strong feeling and could come across as rude (depending on how it is said: quietly/ forcefully, shouting...). "Cut it out","knock it off" both add an extra layer of anger/ irritation/ annoyance towards the person being addressed. The 2 latter are also more commonly used in the U.S.

"Stop it" is written classical language. "Cut it out" is very colloquial spoken speech. "Knock it off" is in the middle between correct written speech and very casual conversational speech.