For words that only have 3 letters, there isn't any 4th letter. By way of example with the word “insert”, there won't be any a lot more letters afterwards and so need to be the word "add". The word “addict” is in the BIP39 wordlist however , you might have https://wearethelist.com/story20923937/considerations-to-know-about-bip39-seed-phrase-generator