The equivalent emf of the combination can be calculated by applying the parallel combination of batteries

$V_{\mathrm{ab}}={\epsilon }_{\mathrm{eq}}=\frac{\frac{{\epsilon }_1}{r_1}+\frac{{\epsilon }_2}{r_2}+\frac{{\epsilon }_3}{r_3}}{\frac{1}{r_1}+\frac{1}{r_2}+\frac{1}{r_3}}$

$\Rightarrow V_{\mathrm{ab}}={\epsilon }_{\mathrm{eq}}=\frac{\frac{2}{2}+\frac{4}{2}+\frac{4}{2}}{\frac{1}{2}+\frac{1}{2}+\frac{1}{2}}$

$\Rightarrow V_{\mathrm{ab}}={\epsilon }_{\mathrm{eq}}=\frac{10}{3} \mathrm{V}$