For words that have only 3 letters, there is no 4th letter. For instance with the word “add”, there isn't any extra letters Later on and thus should be the word "include". The word “addict” is while in the BIP39 wordlist however , you would have used “addi” for “addict”. https://bookmarklethq.com/story18796132/5-simple-techniques-for-bip39-seed-phrase