For words that have only three letters, there isn't a 4th letter. For instance with the word “incorporate”, there aren't any a lot more letters afterwards and so needs to be the word "add". The word “addict” is during the BIP39 wordlist however , you would've used “addi” for “addict”. https://ariabookmarks.com/story4735185/not-known-factual-statements-about-bip39-explained