For words that have only 3 letters, there is absolutely no 4th letter. By way of example with the word “insert”, there isn't any extra letters Later on and so needs to be the word "add". The word “addict” is during the BIP39 wordlist however , you would've utilized “addi” https://bookmarksusa.com/story18867078/the-ultimate-guide-to-bip39-seed-phrase-github