The pH of a solution is a measure of its acidity or basicity, and is defined as the negative logarithm of the hydrogen ion concentration ([H+]). The lower the pH, the higher the [H+], and vice versa.

A pH of 1 means that the [H+] is 10^-1 = 0.1 M. A pH of 3 means that the [H+] is 10^-3 = 0.001 M.

Therefore, an acid with a pH of 1 has a greater [H+] and is therefore more acidic than an acid with a pH of 3.