For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “add”, there isn't any extra letters afterwards and thus needs to be the word "include". The word “addict” is in the BIP39 wordlist however, you might have made use of “addi” for https://socialaffluent.com/story4987040/the-greatest-guide-to-seed-phrase-bip39