Lose 1 signing device and its seed backup but still have its corresponding puclic key backup, you can still recreate and spend from your multisig. Lose the device, its seed and its puclic key and you lose your bitcoin.

Keep a copy of all 3 public keys with every seed backup.

If someone were to gain access to all three of your public keys, they could build your wallet but could not spend from it. This is a privacy issue, not a security one.