Three identical, parallel conducting plates A, .B and C are placed as shown in fig. Switches S1 and  S2 open, and can connect A and C to earth when closed. +Q charge is given to B.

When either a or C is earthed, a parallel plate capacitor is formed with plate B. There will be ±Q charges on inner surfaces. So, a charge of amount +Q flows to earth. When A and C both are earthed together, the plates form two capacitors in parallel. Their capacities will be in the ratio 1:2 and hence they share the charge Q/3 which will pass through S1 and a charge 2 Q/3 which will pass through S2.