For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there won't be any much more letters afterwards and thus should be the word "add". The word “addict” is during the BIP39 wordlist but you would have employed “addi” for “addict”. - https://bookmarkplaces.com/story18972739/getting-my-bip39-words-to-numbers-to-work