Ok, so I'm learning through doing here. I took what I know to be valid xpubs from my three keys and used the derivation path for nested segwit. However, I can't sign from that quorum. I'm thinking this is probably because the xpubs I am using were not generated by the derivation path I specified?
You can't sign because you don't have the private key, So is not because of different xpubs or public keys.