For words that only have 3 letters, there isn't any 4th letter. For instance with the word “add”, there isn't any extra letters Later on and thus must be the word "include". The word “addict” is in the BIP39 wordlist however, you would have used “addi” for “addict”. A seed https://minibookmarking.com/story18920697/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator