For words that only have 3 letters, there isn't any 4th letter. Such as with the word “insert”, there aren't any much more letters Later on and therefore need to be the word "include". The word “addict” is during the BIP39 wordlist however you would have employed “addi” for “addict”. https://yoursocialpeople.com/story4276385/little-known-facts-about-bip39-words