For words that have only 3 letters, there isn't a 4th letter. As an example with the word “add”, there won't be any more letters Later on and therefore need to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've utilised “addi” for “addict”. https://bookmark-search.com/story18752235/fascination-about-bip39-english-words