For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there are no extra letters Later on and so should be the word "incorporate". The word “addict” is inside the BIP39 wordlist however, you would've utilised “addi” for “addict”. By adhering to https://hylistings.com/story20298937/little-known-facts-about-bip39-words