For words that only have 3 letters, there is no 4th letter. By way of example with the word “insert”, there are no extra letters Later on and thus should be the word "add". The word “addict” is from the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://socialtechnet.com/story4257167/little-known-facts-about-bip39-seed-phrase-list