For words that only have 3 letters, there isn't a 4th letter. As an example with the word “add”, there won't be any additional letters afterwards and so has to be the word "include". The word “addict” is during the BIP39 wordlist however, you would have applied “addi” for “addict”. https://modernbookmarks.com/story18646897/little-known-facts-about-bip39-words