For words that have only 3 letters, there's no 4th letter. By way of example with the word “incorporate”, there won't be any additional letters afterwards and therefore have to be the word "insert". The word “addict” is in the BIP39 wordlist however , you would've employed “addi” for “addict”. https://hypebookmarking.com/story18587716/the-5-second-trick-for-ledger-seed-phrase-bip39