For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “include”, there isn't any extra letters Later on and thus need to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would've used “addi” for “addict”. https://hylistings.com/story20299370/a-review-of-bip39-phrase-list