For words that have only 3 letters, there isn't a 4th letter. One example is with the word “add”, there won't be any extra letters Later on and so needs to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have utilised “addi” https://bookmarkfame.com/story18728565/fascination-about-bip39-english-words