For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any extra letters Later on and therefore have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you would've applied “addi” for “addict”. https://bookmarkpath.com/story19570079/little-known-facts-about-bip39-seed-phrase-list