What is the charge on the left side of the plate and how can it be found?

what do you mean by the left side of the plate??

if charge 5Q is on plate A then that means that the whole charge is distributed over that plateequally.

if 3Q is on plaate B then that means -3 Q here will not be distributed equally on B because due to attraction of 5Q and due to repulsion of -2Q charge on plate C most of the negative charges will be on the   left side of the plate B .