For words that have only 3 letters, there is no 4th letter. For example with the word “add”, there isn't any additional letters afterwards and so have to be the word "increase". The word “addict” is during the BIP39 wordlist however , you might have used “addi” for “addict”. You https://ok-social.com/story4400099/5-essential-elements-for-bip39-explained