The pH of a solution is determined using the formula:
\[
\text{pH} = -\log [\text{H}^+]
\]
In this formula, \( [\text{H}^+] \) represents the molar concentration of hydrogen ions (\( \text{H}^+ \)) in the solution.
Hydrochloric acid (HCl) is classified as a strong acid because it undergoes complete dissociation in water. Consequently, the concentration of \( \text{H}^+ \) ions is identical to the initial concentration of the HCl solution.
The provided hydrogen ion concentration is:
\[
[\text{H}^+] = 0.001 \, \text{M} = 10^{-3} \, \text{M}
\]
Substituting this concentration into the pH equation yields:
\[
\text{pH} = -\log (10^{-3})
\]
\[
\text{pH} = -(-3) = 3
\]
Therefore, a 0.001 M hydrochloric acid solution has a pH of \( 3 \).