For words that have only 3 letters, there isn't any 4th letter. Such as with the word “insert”, there won't be any more letters Later on and therefore have to be the word "include". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. A https://dmozbookmark.com/story18854498/fascination-about-bip39-english-words