For words that have only 3 letters, there isn't a 4th letter. For instance with the word “add”, there isn't any much more letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however , you might have made use of “addi” https://bookmarkingdelta.com/story18811747/5-essential-elements-for-bip39-explained