For words that only have 3 letters, there is no 4th letter. For instance with the word “insert”, there aren't any much more letters Later on and thus must be the word "insert". The word “addict” is during the BIP39 wordlist however you would've utilized “addi” for “addict”. If you https://nimmansocial.com/story8594361/new-step-by-step-map-for-bip39-english-words