For words that have only 3 letters, there isn't any 4th letter. For instance with the word “insert”, there aren't any extra letters Later on and therefore must be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would've utilized “addi” for “addict”. By following https://7prbookmarks.com/story18826000/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting