All capacitor are in parallel combination. Also effective area is common area only $$ \begin{aligned} & \Rightarrow C_{e q}=C_1+C_2+C_3 \\ & \Rightarrow C_{e q}=\frac{A \varepsilon_0}{3 d}+\frac{A \varepsilon_0}{3(2 d)}+\frac{A \varepsilon_0}{3(3 d)} \\ & \Rightarrow C_{e q}=\frac{A \varepsilon_0}{3}\left(\frac{11}{6 d}\right) \\ & \Rightarrow C_{e q}=\frac{11 A \varepsilon_0}{18 d} \end{aligned} $$