For words that have only 3 letters, there is not any 4th letter. For example with the word “insert”, there won't be any additional letters Later on and thus have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would've utilized “addi” for “addict”. https://socialwebleads.com/story5074317/facts-about-bip39-seed-phrase-word-list-revealed