For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “include”, there are no much more letters afterwards and therefore have to be the word "include". The word “addict” is from the BIP39 wordlist however you would've used “addi” for “addict”. https://mediasocially.com/story4401258/the-basic-principles-of-bip39-passphrase-generator