For words that have only 3 letters, there isn't a 4th letter. For instance with the word “increase”, there are no additional letters afterwards and therefore needs to be the word "increase". The word “addict” is from the BIP39 wordlist however you would have made use of “addi” for “addict”. https://mnobookmarks.com/story18882808/fascination-about-invalid-bip39-phrase