For words that have only 3 letters, there isn't a 4th letter. For example with the word “insert”, there are no additional letters Later on and therefore should be the word "incorporate". The word “addict” is within the BIP39 wordlist however , you would have used “addi” for “addict”. By https://loanbookmark.com/story18880710/top-guidelines-of-bip39-word-list-pdf