Given:
The concentration of hydrogen ions \( [\text{H}^+] = 1 \times 10^{-3} \, \text{mol/L} \).
Step 1: Use the formula for pH
The pH is calculated using the formula:
\[
\text{pH} = -\log[\text{H}^+]
\]
Step 2: Substitute the given concentration
Substitute \( [\text{H}^+] = 1 \times 10^{-3} \) into the formula:
\[
\text{pH} = -\log(1 \times 10^{-3}) = 3
\]
Answer: The correct answer is option (1): 3.