All three statements provided in the question regarding mercury pollution are true.

1. Gold mining indeed contributes to mercury pollution. Mercury is used in gold mining to separate gold from ore, leading to its release in the environment causing pollution.

2. Coal-based thermal power plants cause mercury pollution because mercury is present in coal. When coal is burned in these plants, mercury is released into the environment.

3. The statement that there`s no known safe level of exposure to mercury is also correct. Mercury is toxic and can cause harm even at low levels of exposure. Chronic exposure to mercury can result in mental and physical health problems.

Therefore, option 3, which suggests that all three statements are accurate, is the correct answer.