For words that have only 3 letters, there is not any 4th letter. For example with the word “add”, there isn't any extra letters afterwards and so has to be the word "increase". The word “addict” is within the BIP39 wordlist however, you might have used “addi” for “addict”. In https://natural-bookmark.com/story18776610/invalid-bip39-phrase-things-to-know-before-you-buy