For words that have only 3 letters, there's no 4th letter. For instance with the word “add”, there won't be any more letters afterwards and thus must be the word "increase". The word “addict” is during the BIP39 wordlist however, you would've applied “addi” for “addict”. A seed phrase, generally https://bookmarkspiral.com/story19017059/fascination-about-bip39-english-words