For words that have only 3 letters, there is not any 4th letter. For instance with the word “add”, there isn't any far more letters Later on and thus needs to be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would've made use of “addi” https://bookmarkangaroo.com/story19589015/a-review-of-bip39-phrase-list