Bip39 seed to private key https://devinzryy552.trexgame.net/comprehending-bip32-vs-bip39-which-one-should-you-make-use-of Even though it uses BIP32 technique you can still import BIP39 seeds into Electrum wallet. Also other than BIp32 There's BIP44 Edition which some wallet use to the era of seeds. For words that have only three letters, there is not any 4th letter. https://my-social-box.com/story5962087/bip39-docs-the-5-second-trick-for-bip39-compatible-wallet-https-dzen-ru-bip39-phrase-com