For words that only have 3 letters, there is not any 4th letter. Such as with the word “include”, there are no far more letters afterwards and thus must be the word "increase". The word “addict” is while in the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://whitebookmarks.com/story19531783/getting-my-bip39-words-to-numbers-to-work