For words that have only 3 letters, there's no 4th letter. By way of example with the word “insert”, there are no far more letters afterwards and so must be the word "insert". The word “addict” is within the BIP39 wordlist however , you might have used “addi” for “addict”. https://ticketsbookmarks.com/story18747889/new-step-by-step-map-for-ledger-seed-phrase-bip39