Then if this seed gets compromised, wouldn’t the hacker use them to recover all the three keys? Although the hacker would still need one more co-signer but wouldn’t that be too risky and probably defeats the whole idea of multi sig?
That is kind of besides the point, since the intention of OP wasn't for one co-signer to have additional security but for the entire system to require the signature of one specific cosigner. Though, there is also a point in that, but the security being provided would be the same as n-of-m multisig, with m distinct entities.Besides, I do agree that using separate seeds would both improve security and reduce the complexity of having to track multiple derivation paths. Wallets like Electrum already makes use of their own versioning system to reduce the chances of user error. It isn't that much more complicated to store a few more seeds anyways.
