For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there aren't any far more letters Later on and so need to be the word "add". The word “addict” is inside the BIP39 wordlist however, you might have made use of https://thekiwisocial.com/story4449352/examine-this-report-on-bip39-mnemonic-phrase-tool