find the capacitance of the arrangement of 4 plates of area A atdistance d in air as shown

Given that separation between each plate is d and area of each plate is A.
Capacitance for parallel plate capacitors with separation d and area of plate, A, is given as
$C=\frac{{\epsilon }_{0}A}{d}$           .....(1)
In the given arrangement, lower face of plate 1 and upper face of plate 2 form a capacitor, lower face of plate 2 and upper face of plate 3 form another capacitor and lower phase of plate 3 and upper face of plate 4 forms the third capacitor. Capacitance of each of the three capacitors is given by (1). 

Since the three capacitors are connected in parallel, their net capacitance is
$$\begin{gathered} C_{net}=\frac{{\epsilon }_{0}A}{d}+\frac{{\epsilon }_{0}A}{d}+\frac{{\epsilon }_{0}A}{d} \\ \Rightarrow C_{net}=\frac{3{\epsilon }_{0}A}{d} \end{gathered}$$