For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “insert”, there aren't any more letters afterwards and so need to be the word "include". The word “addict” is during the BIP39 wordlist however , you would've used “addi” for “addict”. On https://bookmarklinking.com/story4970905/getting-my-bip39-words-to-numbers-to-work