It's easy to keep secret when you only one who knows truth. It's also easy for two, because you immediately know who to blame if secret is revealed, so you can keep pressure. But situation changes for three, because you cannot know who revealed secret, so it's much easier to crack.

Imagine, I will report today that I'm able to determine temperature of the physical vacuum (e.g. using double slit experiment), and then able to drop it, i.e. cool vacuum below absolute zero. Will you trust me? Of course, not. Then when you will trust me?