For words that have only 3 letters, there's no 4th letter. By way of example with the word “incorporate”, there are no far more letters Later on and therefore must be the word "add". The word “addict” is inside the BIP39 wordlist however you might have applied “addi” for “addict”. https://fatallisto.com/story8572353/fascination-about-bip39-english-words