For words that have only three letters, there is not any 4th letter. Such as with the word “include”, there are no more letters afterwards and therefore should be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would have made use of “addi” https://naturalbookmarks.com/story19075370/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition