All three statements are correct, reflecting critical environmental concerns associated with coal usage in India.

1. Coal ash, a byproduct of coal combustion, indeed contains heavy metals like arsenic, lead, and mercury, which pose significant health risks and environmental hazards, particularly when mishandled or improperly disposed of.

2. Coal-fired power plants are significant sources of air pollutants, including sulfur dioxide (SO2) and nitrogen oxides (NOx), which contribute to acid rain and smog formation, impacting air quality and public health.

3. Indian coal is characterized by high ash content, generally exceeding global averages, leading to reduced thermal efficiency and greater environmental impact from coal combustion.

Together, these statements highlight the environmental and health implications of coal dependence in India, emphasizing the need for cleaner energy alternatives—a relevant issue for UPSC aspirants as it underlines the intersection of energy policy, public health, and environmental sustainability.