For words that only have 3 letters, there isn't a 4th letter. As an example with the word “add”, there won't be any a lot more letters afterwards and so have to be the word "include". The word “addict” is in the BIP39 wordlist however , you would have applied https://tvsocialnews.com/story4341728/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition