For words that have only 3 letters, there is not any 4th letter. For instance with the word “insert”, there aren't any a lot more letters afterwards and thus have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would've used “addi” for “addict”. https://bookmarkspecial.com/story18971278/a-review-of-bip39-phrase-list