HCl <--> H(+) + Cl(-)
HCl is a strong acid, so it completely dissociates (meaning the reaction goes all the way to the right).
Therefore, since the stochiometric ratio between HCl and H(+) is 1:1, so [H+] is (1 x 10^-3)
pH = -log[H+] = -log(1 x 10^-3) = 3
So pH = 3
... hope that helps