For words that only have 3 letters, there isn't a 4th letter. For example with the word “include”, there are no extra letters afterwards and so need to be the word "add". The word “addict” is from the BIP39 wordlist however, you might have made use of “addi” for “addict”. https://bookmarkblast.com/story18847318/not-known-facts-about-bip39-english-words