Three large, parallel conducting plates P, Q and R are placed horizontally such  that the plates P and R are rigidly fixed and earthed and the plate Q is given some charge. Under the influence of the electrostatic and gravitational forces, the plate Q may stay in equilibrium 
 
1) midway between P and R                      2) if it is closer to P than to R
3) if it is closer to R than to P                    4) which is always unstable

Since P and R are earthed and so they can be assumed to be connected to each other. Hence, $\text{'}P+Q\text{'}$  and $\text{'}Q+R\text{'}$  are two capacitors with the same potential difference. If Q is closer to P than to R then the capacitance  $C_{PQ}$  is  $>C_{QR}$. The upper surface of Q will have greater charge than the lower surface. As the force of attraction between the plates of a capacitor is proportional to  $Q^2$  so, there will be a net upward force  on Q which can  balance its weight.