For words that only have 3 letters, there is absolutely no 4th letter. Such as with the word “insert”, there won't be any a lot more letters afterwards and so have to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would have applied “addi” https://cyberbookmarking.com/story19535305/the-greatest-guide-to-seed-phrase-bip39