When the pH decreases from 6 to 3, the hydrogen ion, H+, increases by what?

pH decreases as the hydrogen ion concentration increases.

Explanation:

When there is a decrease in pH, that is pH decreases from 6 to 3 then the acidity increases.

That is the pH is between 1 to 7 then it is acidic

When the pH is 7 then it is neutral

When the pH is between 7 to 14 then it is basic

As the H⁺ ion concentration increases, then the pH value decreases, here pH decreases from 6 to 3.

So the concentration of Hydrogen ion increases, pH decreases.