What if one of them is incapacitated and forgets where they kept ther own key, or is kidnapped , or even die? How would the remaining two guys access the coins?
For 3-of-3, there's nothing they can do since the access is not enforced by the wallet that they are using (
not client-side) or a central authority.
It's what's indicated in the "
redeem script" of each UTXO (
Unspent Transaction Output) associated with their MultiSig wallet.
For 3-of-3 MultiSig, the redeem script is something like this:
OP_3 (required number of signatures) <pubKey1> <pubKey2> <pubKey3> OP_3 (number of cosigners) OP_CHECKMULTISIG
If they can't fulfil number of required signatures indicated in the script which only the private key pairs of the indicated publics key can create,
the UTXO can't be spent and no Bitcoin technical support or Bitcoin developer can help them to gain access to their bitcoins.
If it's a three-key wallet, the three key holders must jointly unlock it. Right?
Depends on the script used, based on the example above, it's more common to use something like this:
OP_2 <pubKey1> <pubKey2> <pubKey3> OP_3 OP_CHECKMULTISIG
The "
OP_2" on the start indicates that it only requires 2 signatures (
2-of-3 MultiSig).
In that case, the "
remaining two guys" still have access to their bitcoins.