For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “increase”, there isn't any more letters afterwards and therefore should be the word "add". The word “addict” is during the BIP39 wordlist however you would've made use of “addi” for “addict”. https://bookmarkinglog.com/story18801076/fascination-about-invalid-bip39-phrase