For words that have only 3 letters, there isn't a 4th letter. As an example with the word “increase”, there won't be any much more letters afterwards and therefore needs to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've used “addi” for “addict”. https://postheaven.net/tiablexcen/mastering-your-copyright-protection-the-essential-overview-to-bip39-passphrases