For words that have only 3 letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any more letters Later on and so should be the word "add". The word “addict” is within the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://free-bookmarking.com/story19539058/bip39-seed-phrase-fundamentals-explained