For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there isn't any additional letters Later on and so has to be the word "incorporate". The word “addict” is during the BIP39 wordlist however, you would have used “addi” for “addict”. https://bookmarksfocus.com/story4400081/top-latest-five-bip39-words-urban-news