For words that only have 3 letters, there isn't any 4th letter. For example with the word “add”, there won't be any more letters Later on and so must be the word "incorporate". The word “addict” is during the BIP39 wordlist however , you might have employed “addi” for “addict”. https://lingeriebookmark.com/story8866591/5-essential-elements-for-bip39-explained