In such type of plate questions ,I have been told that when any one of the plates is grounded then the charge flown out would be the sum of the charges on all plates Why is this so
Secondly Why the charge on the outer faces of the plates at the ends is zero and do not have a reasonable explation
The answer to your first question follows on from the answer to your second question.
The system of three plates will try and minimize the stored energy.
If there are charges on the outer surface of the two outer plates then there will be an electric field outside the confines of the three plates.
That electric field is a store of energy.
Having no changes on the outside of the outer plates means that there is no electric field outside the three plates.
So the right-hand plate must have $+70\,\mu\rm C$ on its left face and $0\,\mu\rm C$ on its right face.
The middle plate must have $-70\,\mu\rm C$ on its right face and $+30-(-70) = +100\,\mu\rm C$ on its left face.
The left-hand plate must have $-100\,\mu\rm C$ on its right face and $0\mu\rm C$ on its left face, thus it has lost $+20+100=120 = 20+30+70\,\mu\rm C$ to ground.
