For words that have only 3 letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any much more letters Later on and thus has to be the word "add". The word “addict” is during the BIP39 wordlist however, you might have used “addi” https://bookmarkgenious.com/story18976214/facts-about-bip39-seed-phrase-word-list-revealed