Doing anything for three hours is a mistake. I've done three hour classes before and don't like them. Its usually to please administrative demands and has little to no educational merit. There are things you can do to make it less taxing on both you and your students. Sandwiching different kinds of activities together is a good idea. Since you're going to be spending so much time with them its a good idea to be stimulating their curiosity to keep the interest level high. Have you got any home movies of your family back home.