For words that have only 3 letters, there isn't a 4th letter. For instance with the word “add”, there are no extra letters Later on and thus have to be the word "increase". The word “addict” is during the BIP39 wordlist however , you might have applied “addi” for “addict”. https://myeasybookmarks.com/story4496481/facts-about-bip39-seed-phrase-word-list-revealed