I live in Montana so maybe my opinion is skewed as some things I take for granted might really turn someone else's crank. But my opinion is you won't want to leave Yellowstone after only 3 days. And Glacier should maybe be a whole other trip because that end of the state deserves a lot of time too. I like what others have said about Bozeman, Virginia City, Ennis, Lewis & Clark caverns, Helena. You will spend a whole lot more time "doing" and less time driving. Montana is a big state- you just might have to come back a second time- and that's a good thing.