How is it possible that a 2 out of 3 setup only needs one signature? Please let me know if you need more details.
I am having a hard time understanding you. But from your story, you already signed the tx with wallet A and B and that is the 2 signatures that is required to broadcast the tx out of the 3 keys. In a 2 of 3 multisig wallet, you only need two keys to sign and broadcast your tx and i believe that is what you have done here, except i myself is missing something.