the remaining three words out of the eighteen could be dictionary brute force attacked
Its 3 out of 24.
it is not a hard task and would be easier and faster than cracking some missing characters in a private key.
3 words (with a character set of 2048) conforms
8.589.934.592 possibilities.
Where 3 missing characters of a private key (with a char set of 16, assuming hex represantation) conforms to
4.096 possibilities.
So, no it would NOT be easier to crack a few words in a seed than a few chars in a priv key.