For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “include”, there won't be any additional letters Later on and therefore should be the word "add". The word “addict” is in the BIP39 wordlist however, you would have applied “addi” for “addict”. https://bookmarksbay.com/story18865852/little-known-facts-about-bip39-words