For words that only have 3 letters, there is no 4th letter. As an example with the word “insert”, there isn't any additional letters Later on and so should be the word "include". The word “addict” is while in the BIP39 wordlist however , you would've used “addi” for “addict”. https://bookmarkcolumn.com/story19415819/examine-this-report-on-bip39-mnemonic-phrase-tool