For words that have only three letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any a lot more letters Later on and thus must be the word "insert". The word “addict” is within the BIP39 wordlist however , you would've made use of https://bookmarkize.com/story18870686/invalid-bip39-phrase-things-to-know-before-you-buy