I have to agree with Buck, and in fact, in some ways its even a bit worse.

The second day's 600 mile drive is a killer. Its practically a full day drive to Duluth (although I will note that US-53 is now a 4 lane expressway all the way to Superior) and that alone with take you 8 hours. Duluth to Grand Marias is only another 150 miles, but it is along an incredibly scenic highway, that is also slow going. You could easily spend a full day making this drive (in fact, just a few weeks ago I was up there and leaving from Duluth, I barely made it halfway to Grand Marias in one day.) Even if you skip all of the amazing attractions along this route, like Split Rock Lighthouse and Gooseberry Falls, just to name a couple, its still a route that is going to take you more than 3, and likely closer to 4 hours.

I have to agree, adding a half day is the only way you'll even be able to think about making this a fun trip, and a full 3rd day would be highly recommended.