For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “incorporate”, there won't be any additional letters Later on and so must be the word "insert". The word “addict” is inside the BIP39 wordlist however, you might have used “addi” for “addict”. https://socialinplace.com/story4270969/top-guidelines-of-bip39-word-list-pdf