When the hydrogen ion concentration $\left[{\mathrm{H}}^{+}\right]$ changes by a factor of 1000, the value of P^H of the solution

${{\mathrm{P}}^{\mathrm{H}}}_1=-\log {\left({\mathrm{H}}^{+}\right)}_1$
If I will take ${\left({\mathrm{H}}^{+}\right)}_1={10}^{-4}\text{ then }{{\mathrm{P}}^{\mathrm{H}}}_1=4$
${{\mathrm{P}}^{\mathrm{H}}}_2-\log {\left({\mathrm{H}}^{+}\right)}_2=-\log \left({10}^{-4}\times {10}^3\right)=-\log {10}^{-1}=\mathbf{1}$
P^H decreases by 3 units.