For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there are no extra letters Later on and therefore should be the word "incorporate". The word “addict” is during the BIP39 wordlist but you would've applied “addi” for “addict”. In this https://thebookmarklist.com/story19092283/the-definitive-guide-to-bip39-seed-phrase