For words that only have 3 letters, there is no 4th letter. One example is with the word “incorporate”, there aren't any additional letters Later on and thus should be the word "insert". The word “addict” is during the BIP39 wordlist however, you would've applied “addi” for “addict”. This text https://bookmarkingbay.com/story19021972/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39