For words that have only 3 letters, there isn't a 4th letter. For example with the word “add”, there isn't any additional letters afterwards and so have to be the word "include". The word “addict” is during the BIP39 wordlist however , you would have used “addi” for “addict”. On https://thesocialcircles.com/story4459704/top-latest-five-bip39-words-urban-news