For words that have only 3 letters, there's no 4th letter. As an example with the word “incorporate”, there won't be any extra letters Later on and so has to be the word "add". The word “addict” is during the BIP39 wordlist but you might have employed “addi” for “addict”. https://210list.com/story19430576/top-guidelines-of-bip39-word-list-pdf