For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “include”, there isn't any additional letters Later on and so have to be the word "add". The word “addict” is in the BIP39 wordlist however, you would've utilized “addi” for “addict”. https://rotatesites.com/story20197012/not-known-facts-about-bip39-english-words