For words that have only 3 letters, there isn't any 4th letter. As an example with the word “add”, there won't be any much more letters Later on and thus have to be the word "include". The word “addict” is from the BIP39 wordlist however, you might have applied “addi” https://funny-lists.com/story19999211/not-known-factual-statements-about-bip39-explained