For words that only have 3 letters, there isn't a 4th letter. Such as with the word “insert”, there won't be any much more letters afterwards and therefore has to be the word "incorporate". The word “addict” is from the BIP39 wordlist however, you would have applied “addi” for “addict”. https://sound-social.com/story8819914/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39