For words that have only 3 letters, there's no 4th letter. Such as with the word “incorporate”, there aren't any extra letters Later on and so have to be the word "add". The word “addict” is during the BIP39 wordlist but you might have made use of “addi” for “addict”. https://expressbookmark.com/story19023054/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool