For words that only have 3 letters, there is not any 4th letter. One example is with the word “add”, there won't be any additional letters Later on and thus should be the word "insert". The word “addict” is during the BIP39 wordlist however , you would have used “addi” https://agendabookmarks.com/story18757225/not-known-facts-about-bip39-english-words