For words that only have 3 letters, there isn't a 4th letter. Such as with the word “add”, there are no a lot more letters afterwards and therefore should be the word "include". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. On this https://writeablog.net/ipennywpzc/h1-b-opening-the-power-of-bip39-exactly-how-to-produce-and-use-mnemonic