In the context of the Moon landing conspiracy theories, first-party evidence is any evidence provided by NASA or the United States Government. Second-party evidence is evidence provided by the conspiracy theorists themselves. Third-party evidence is evidence provided by people or organizations that have nothing to do with NASA, the USA, or the conspiracy theorists whatsoever.
I’ll be posting some of this third party evidence soon. Stay tuned, examine the evidence, and then decide for yourself 🙂