For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “add”, there aren't any additional letters afterwards and therefore should be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would have used “addi” for “addict”. Locating https://single-bookmark.com/story19190814/how-much-you-need-to-expect-you-ll-pay-for-a-good-valid-bip39-seed-phrase