Suppose three men rob a bank.
What legal mechanism prevents a prosecutor from compelling testimony from each of them regarding the other two, thereby having two witnesses against each defendant?
They are claiming attorney-client privilege, all while not acting as an attorney, merely as advisor.
The privilege against self incrimination goes to all crimes. In your hypothetical, all three are under the shadow of criminal conspiracy, so all three have a basis to assert the privilege to not testify.