For words that have only 3 letters, there is not any 4th letter. For instance with the word “include”, there won't be any more letters afterwards and so has to be the word "insert". The word “addict” is from the BIP39 wordlist however you would have employed “addi” for “addict”. https://mysocialquiz.com/story4648053/not-known-factual-statements-about-bip39-explained