The correct option is "Only two" statements are correct. Statements 1 and 2 correctly identify gold mining and coal-based thermal power plants as significant sources of mercury pollution, which is crucial in understanding environmental health issues and industrial practices. Statement 3, however, while referring to the high toxicity of mercury, can be nuanced: while low levels of exposure may not exhibit immediate effects, the absence of a "safe" threshold is a subject of ongoing research and debate within toxicology. Thus, it's vital for UPSC aspirants to grasp the multifaceted implications of mercury pollution on both ecosystems and human health.