For words that have only 3 letters, there is no 4th letter. For instance with the word “insert”, there isn't any much more letters afterwards and so should be the word "incorporate". The word “addict” is while in the BIP39 wordlist however , you would have applied “addi” for “addict”. https://socialwebconsult.com/story4196494/fascination-about-invalid-bip39-phrase