${\text{V}}_{\text{A}}=\frac{1}{4\pi {\epsilon }_0}\left\{\frac{{\text{q}}_{\text{A}}}{\text{a}}+\frac{{\text{q}}_{\text{B}}}{\text{b}}+\frac{{\text{q}}_{\text{C}}}{\text{c}}\right\}$

     $=\frac{4\pi }{4\pi {\epsilon }_0}\left\{\frac{{\text{a}}^2\sigma }{\text{a}}-\frac{{\text{b}}^2\sigma }{\text{b}}+\frac{{\text{c}}^2\sigma }{\text{c}}\right\}$

$$\begin{gathered} {\text{V}}_{\text{A}}=\frac{1}{{\epsilon }_0}\left\{\frac{{\text{a}}^2\sigma }{\text{a}}-\frac{{\text{b}}^2\sigma }{\text{b}}+\frac{{\text{c}}^2\sigma }{\text{c}}\right\} \\ \Rightarrow {\text{V}}_{\text{A}}=\frac{1}{{\epsilon }_0}\left\{\text{a}\sigma -\text{b}\sigma +\text{c}\sigma \right\}=\frac{2\mathrm{a}\sigma }{{\epsilon }_0} \end{gathered}$$

$$\begin{gathered} {\text{V}}_{\text{B}}=\frac{1}{{\epsilon }_0}\left\{\frac{{\text{a}}^2\sigma }{\text{b}}-\frac{{\text{b}}^2\sigma }{\text{b}}+\frac{{\text{c}}^2\sigma }{\text{c}}\right\} \\ {\text{V}}_{\text{B}}=\frac{1}{{\epsilon }_0}\left\{\frac{{\text{a}}^2\sigma }{\text{b}}-\text{b}\sigma +\text{c}\sigma \right\} \\ {\text{V}}_{\text{B}}=\frac{\mathrm{a\sigma }}{{\epsilon }_0}\left\{\frac{\text{a}}{\text{b}}-1\right\} \end{gathered}$$

$$\begin{gathered} {\text{V}}_{\text{C}}=\frac{1}{{\epsilon }_0}\left\{\frac{{\text{a}}^2\sigma }{\text{c}}-\frac{{\text{b}}^2\sigma }{\text{c}}+\frac{{\text{c}}^2\sigma }{\text{c}}\right\} \\ {\text{V}}_{\text{C}}=\frac{\sigma }{{\epsilon }_0}\left\{\frac{{\text{a}}^2{\text{-b}}^2}{\mathrm{a}+\mathrm{b}}+\mathrm{c}\right\} \\ {\text{V}}_{\text{C}}=\frac{\sigma }{{\epsilon }_0}\left\{\text{a-b}+\mathrm{c}\right\} \\ {\text{V}}_{\text{C}}=\frac{2\sigma \mathrm{a}}{{\epsilon }_0} \end{gathered}$$

${\mathrm{V}}_{\mathrm{A}}={\mathrm{V}}_{\mathrm{C}}>{\mathrm{V}}_{\mathrm{B}}$