For words that have only 3 letters, there is not any 4th letter. One example is with the word “insert”, there aren't any extra letters afterwards and so need to be the word "include". The word “addict” is during the BIP39 wordlist however you might have utilized “addi” for “addict”. https://iwanttobookmark.com/story18916828/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator