For words that only have 3 letters, there isn't a 4th letter. By way of example with the word “insert”, there isn't any extra letters afterwards and therefore needs to be the word "include". The word “addict” is within the BIP39 wordlist however you would've utilised “addi” for “addict”. BIP39 https://seolistlinks.com/story20190628/little-known-facts-about-bip39-words