For words that have only 3 letters, there's no 4th letter. For instance with the word “insert”, there are no additional letters Later on and so need to be the word "incorporate". The word “addict” is while in the BIP39 wordlist however you would've made use of “addi” for “addict”. https://maximusbookmarks.com/story19000110/not-known-facts-about-bip39-english-words