For words that have only 3 letters, there is absolutely no 4th letter. By way of example with the word “insert”, there are no additional letters Later on and thus must be the word "include". The word “addict” is while in the BIP39 wordlist however, you would have used “addi” https://bookmarkmoz.com/story18866946/not-known-factual-statements-about-bip39-explained