For words that have only 3 letters, there isn't any 4th letter. For example with the word “add”, there isn't any far more letters afterwards and so has to be the word "add". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. This information https://peakbookmarks.com/story19031821/not-known-facts-about-bip39-english-words