For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there isn't any extra letters afterwards and thus need to be the word "incorporate". The word “addict” is in the BIP39 wordlist however you would have utilised “addi” for “addict”. Mnemonic words https://bookmarkspy.com/story20186731/examine-this-report-on-bip39-mnemonic-phrase-tool