$\begin{array}{l}{\mathrm{H}}_2\mathrm{A}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{HA}}^{-}\\ \because y\text{ is very less so }x+y\approx x\therefore K_{a_1}=\frac{x^2}{c-x}\end{array}$

At equilibrium 

$\text{ C- }Xx+yx-y{K}_{a_2}=y={10}^{-12};p{K}_{a_2}=12$

$\begin{array}{l}{\mathrm{HA}}^{-1}\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{A}}^{2-}\\ \text{ at equilibrium }x-yx+yy\\ {\mathrm{pK}}_{{\mathrm{a}}_2}-{\mathrm{pK}}_{{\mathrm{a}}_1}=12-5=7\\ \mathrm{pH}=\frac{1}{2}\left[{\mathrm{pK}}_{{\mathrm{a}}_1}-\log C\right]\\ 3=\frac{1}{2}\left[{\mathrm{pK}}_{{\mathrm{a}}_1}+1\right]{\mathrm{pK}}_{{\mathrm{a}}_1}=5\end{array}$