For words that have only 3 letters, there's no 4th letter. For instance with the word “add”, there won't be any far more letters afterwards and thus have to be the word "insert". The word “addict” is from the BIP39 wordlist however, you would've made use of “addi” for “addict”. https://indexedbookmarks.com/story18919544/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator