For words that have only three letters, there's no 4th letter. For instance with the word “insert”, there aren't any more letters Later on and thus have to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would've used “addi” for “addict”. Mnemonic words illustrations https://bookmarkcolumn.com/story18644920/the-2-minute-rule-for-bip39-all-words