For words that have only 3 letters, there is not any 4th letter. By way of example with the word “add”, there aren't any additional letters Later on and so must be the word "include". The word “addict” is within the BIP39 wordlist however you would have employed “addi” for https://advicebookmarks.com/story26505557/facts-about-bip39-seed-phrase-word-list-revealed