1. Main drains construction in Harappa: The main drains in Harappa were indeed constructed using bricks set in mortar and were mostly covered and hidden underground. This indicates an advanced municipal system and knowledge of construction techniques.
2. The Great Bath of Mohenjo-daro: The Great Bath is widely regarded as the earliest public water tank of the ancient world, showcasing the importance of ritualistic cleaning in the culture of the Indus Valley Civilization.
3. Settling pools and traps in the drainage system: The drainage systems in Harappa included small settling pools and traps to collect sediment and other materials, demonstrating a sophisticated approach to managing waste and maintaining cleanliness.
All three statements are correct, thus the correct answer is Option D.