For words that have only 3 letters, there isn't a 4th letter. By way of example with the word “add”, there won't be any more letters afterwards and thus has to be the word "include". The word “addict” is in the BIP39 wordlist however , you would have made use https://webnowmedia.com/story4297598/the-2-minute-rule-for-bip39-all-words