For words that have only 3 letters, there is not any 4th letter. Such as with the word “add”, there aren't any much more letters afterwards and so has to be the word "insert". The word “addict” is within the BIP39 wordlist however, you would've utilized “addi” for “addict”. This https://bookmarkwuzz.com/story18813013/a-review-of-bip39-phrase-list