For words that have only 3 letters, there is no 4th letter. For instance with the word “add”, there are no additional letters Later on and therefore has to be the word "include". The word “addict” is inside the BIP39 wordlist however you would have used “addi” for “addict”. Finding https://bookmarklayer.com/story18828186/an-unbiased-view-of-bip39-seed-phrase-github