For words that have only 3 letters, there isn't a 4th letter. For example with the word “insert”, there won't be any additional letters afterwards and thus has to be the word "include". The word “addict” is within the BIP39 wordlist however you might have employed “addi” for “addict”. Various https://letsbookmarkit.com/story19572106/phrases-with-bit-an-overview