For words that have only 3 letters, there isn't a 4th letter. As an example with the word “insert”, there won't be any additional letters afterwards and therefore need to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. Try https://minibookmarking.com/story19721538/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool