For words that have only 3 letters, there's no 4th letter. For example with the word “add”, there are no additional letters afterwards and so needs to be the word "insert". The word “addict” is during the BIP39 wordlist however you would have made use of “addi” for “addict”. Getting https://www.livebinders.com/b/3631513?tabid=7dd978d9-81c0-05c0-ecba-7d3b4f616e73