For words that have only 3 letters, there is no 4th letter. For instance with the word “add”, there isn't any far more letters Later on and so needs to be the word "insert". The word “addict” is while in the BIP39 wordlist but you might have made use of https://pr1bookmarks.com/story19018637/getting-my-bip39-words-to-numbers-to-work