Which three countries have refused to join the NPT?

Could you please elaborate on the specific paragraph you're referring to, as the acronym "NPT" could potentially stand for multiple international treaties or agreements? However, assuming "NPT" stands for the Nuclear Non-Proliferation Treaty, I'll proceed with a question based on that assumption. Which three countries, to date, have notably refused to accede to the Nuclear Non-Proliferation Treaty (NPT), despite its widespread international recognition and adherence as a cornerstone of global nuclear disarmament and non-proliferation efforts? Are there any particular reasons or motivations behind their decisions to remain outside of this critical treaty framework?