the problem with that method is that it is not an encryption technique and also you will have to have 3 completely separate, isolated and secure places to store each part at and it is not easy to have it. if you are just storing all at home, it is not safe and you can't trust any other person to store it with them either.

adding an extra string (known as passphrase in BIP39) could be a good idea to add another layer of security but i personally prefer simply encrypting the whole thing with AES using a very strong password and then storing that single result in a safe place and the password of it in another.