For words that have only 3 letters, there is no 4th letter. For example with the word “insert”, there are no much more letters afterwards and so should be the word "include". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. Try to be https://infopagex.com/story4304073/not-known-factual-statements-about-bip39-explained