For words that have only 3 letters, there isn't a 4th letter. Such as with the word “insert”, there are no far more letters Later on and so has to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have made use of “addi” https://lingeriebookmark.com/story9488365/new-step-by-step-map-for-ledger-seed-phrase-bip39