When a charge of amount Q is given to an isolated metal plate X of surface area A, its surface charge density becomes σ1. When an isolated identical plate Y is brought close to X, the surface charge density on X becomes σ2. When y is earthed, the surface charge density becomes σ3.
As the plate X is isolated so the charge remains constant on the both surface of X.
As charge Q is distribute into two surfaces of X so the charge density is σ1=QA+A=Q2A
When another isolated plate Y is placed closed to X , the charge density of X will remain same. i.e, σ2=σ1
When Y is earthed, the system acts like a parallel plate capacitor. The charge will from near surface of X to Y. The charge will remain only on the one surface.
now charge density is σ3=QA