For words that only have 3 letters, there is not any 4th letter. For example with the word “insert”, there isn't any much more letters afterwards and therefore must be the word "increase". The word “addict” is in the BIP39 wordlist however, you would have employed “addi” for “addict”. - https://mirrorbookmarks.com/story19078056/new-step-by-step-map-for-bip39-english-words