To get the stake keys, each of you can use the “Mnemonic Method” as described in the coincashew guide to extract your own stake keys from Daedalus (or Yoroi, that’s what I used). You will need to have the 15 or 24 word recovery code. I currently don’t have a hardware wallet, so I can’t tell you if this method would work for that as well.
Once you have the stake keys, you’ll need to register the address as well (with the 20 ADA deposit) an get the stake certificates.
It’s recommended that these steps above are done on air-gapped machines as the keys (vkey, skey) and certs should be stored there.
Move these stake certificates and the verification keys (vkeys) to your block producer node and create your pool registration transaction again. Since you’re already operational, this time around you could set your “poolDeposit” to zero as you should have already deposited the 500 ADA the first time around
Once you have your raw transaction, each of you will need to sign the transaction with the skey. I believe you can each sign it separately on separate air-gapped machines. A signs, sends signed transaction file to B, B signs, B sends newly signed transaction file to C, C signs, move final signed transaction back to block-producer node and submit it. I think this is the recommended way as you don’t need to share your private keys with each other.
As you can tell the steps above require a lot of hands and passing around the transaction file a lot. It’s also “time sensitive” in the sense that your transaction TTL may expire during the time you’re waiting for everyone to sign. So we didn’t do it this way. Instead, we trusted each other and saved our private keys on a single air-gapped machine and signed it in one go.
Hope this helps!