For words that have only 3 letters, there's no 4th letter. For example with the word “add”, there are no additional letters Later on and thus has to be the word "add". The word “addict” is during the BIP39 wordlist however you would've utilised “addi” for “addict”. Mnemonic words illustrations https://bookmarksbay.com/story18866532/new-step-by-step-map-for-bip39-english-words