For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there aren't any much more letters Later on and so need to be the word "include". The word “addict” is from the BIP39 wordlist however, you would've utilised “addi” for “addict”. https://bookmarksusa.com/story18866628/details-fiction-and-bip39