In classical electromagnetics, charges flow from + to -. When the spheres touch, the charges re-arrange to get to the lowest energy state possible. Since there is - 3mC on one sphere and +5mC on the other, + 3mC of charge will flow to the first sphere, reducing the net charge to zero. That leaves +2 mC on the second sphere, but that is not the lowest energy state for the system. Since the first sphere is uncharged, charge can move from the second sphere to the first. So 1 mC of charge will flow from the second sphere giving each sphere 1mC of charge. This is the lowest energy state possible for the the system.