The pH of a 0.001 M HCL solution is 3. It is equivalent to minus logarithm of the hydronium ion concentration, which is 0.001.
HCL stands for hydrogen chloride. It ionizes entirely in water, creating an aqueous solution that contains positively charged hydronium ions and negatively charged chloride ions. This solution acts as a strong acid. Water ionizes into negatively charged hydroxide ions and positively charged hydronium ions. But, the concentration of both of these ions are so small that they are considered insignificant, especially in the case of a strong acid based on Le Chatelier's principle. Thus, only the concentration of HCL's hydronium ions are considered for pH calculation of the aqueous HCL solution.