The same way you cannot prevent someone from reading the amplitude of the EM wave you send through generic wired or wireless media?
The trick has always been in encrypting the aggregate information. But, I am sure that quantum cryptography will be a completely different ball game compared to what we have now.
No, this is wrong; pejoculant has it right above: the act of measuring the spin state destroys the superposition. It's not physically possible for three observers to see the entanglement at once. There's no cryptography required.
The trick has always been in encrypting the aggregate information. But, I am sure that quantum cryptography will be a completely different ball game compared to what we have now.