Since the teacher has said to use quantitative observation, density is the easiest measurement to make. Aluminum has a density of 2.7 g/mL. Brass has a density of 8.73 g/mL. Copper has a density of 8.96 g/mL. As long as the mass and volume measurements are accurate (water displacement can be used for volume measurements), the resulting density values can be distinguished.
It may be possible to distinguish these three metals visually, based on color, but the instructions have stated to use quantitative observation.