For words that have only 3 letters, there's no 4th letter. For instance with the word “add”, there aren't any much more letters afterwards and therefore have to be the word "include". The word “addict” is from the BIP39 wordlist however , you would've used “addi” for “addict”. Mnemonic words https://bookmarkangaroo.com/story19259071/facts-about-bip39-seed-phrase-word-list-revealed