For words that have only 3 letters, there isn't a 4th letter. By way of example with the word “insert”, there isn't any additional letters afterwards and therefore has to be the word "incorporate". The word “addict” is inside the BIP39 wordlist but you might have used “addi” for “addict”. https://bookmarkrange.com/story20234574/the-5-second-trick-for-ledger-seed-phrase-bip39