Bip39 mnemonic checksum https://www.hometalk.com/member/176312665/elva1377050 While it takes advantage of BIP32 system it is possible to still import BIP39 seeds into Electrum wallet. Also besides BIp32 You can find BIP44 version which some wallet use for that generation of seeds. For words that have only 3 letters, there isn't a 4th https://mariobrpx591.cavandoragh.org/getting-a-bip39-phrase-in-python-a-comprehensive-tutorial