I'm with the frog on this one. In light aircraft 4.5 to 5 degrees for the slope during basic training, using the runway shape to judge height on base or final. At night, the runway edge lights and threshold lights at each end should give the same picture. Any PAPI (or LITAS which we still have) to be a back up to the picture.
Teaching 3 degree approaches should be during the commercial training phase, or as advanced training for PPLs, at a suitably long runway. If we used 3 degrees on our prevailing straight in, we'd be around 700 feet above a built up area outside our ATZ, in Class G, and we don't have any instrument approaches as an excuse for being so low. That would also put us too close to the trees and steel rugby posts on very short final, culminating in a long landing on our relatively short tarmac.