Personally I’d use three applications.
Generally speaking in the world of development you would have separate Keys/Configs for dev, testing and production.
What @3ventic suggests is sound, but it means you leave in code in production that could err and send a user to the wrong domain and half the time the code is sat there slowing your website down as it does nothing but interrogate the state parameter for “where should I send this guy” when the redirect URI and Application Keys already decided this for you.
So three applications is the way to go in my opinion