How far behind the shoulder did you hit it? Based on the bear I've taken and field dressed I'm of the opinion the gut pile on a bear is smaller than on a deer. So if you are a little far back it is easy to miss the lungs and get into less vital organs. Bear need to be shot right at the edge of the leg with an arrow or in the case of a rifle, in the shoulder itself.

If he is bad hit, he will eventually head for water. If you lose the trail check the local water sources and see if you can pick it back up. But after 3 days, that bear is probably quietly rotting away somewhere.