For words that only have 3 letters, there isn't a 4th letter. For instance with the word “include”, there aren't any much more letters afterwards and so need to be the word "increase". The word “addict” is from the BIP39 wordlist but you would've applied “addi” for “addict”. This article https://bookmark-group.com/story4570565/not-known-factual-statements-about-bip39-explained