For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any extra letters Later on and so have to be the word "include". The word “addict” is during the BIP39 wordlist however, you would've employed “addi” for “addict”. Artwork is https://bookmark-nation.com/story18648384/the-definitive-guide-to-bip39-seed-phrase