For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any much more letters Later on and thus must be the word "insert". The word “addict” is from the BIP39 wordlist however you might have utilized “addi” for “addict”. https://baidubookmark.com/story18693094/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool