Since the spheres are connected by a conducting wire, they will share charge and be at the same potential. The external charge q breaks the symmetry and induces charges on the spheres due to electrostatic induction.
Let the induced charges on spheres A, B, and C be q_A, q_B, and q_C. The total induced charge on all three spheres must be zero (as they were neutral initially and only affected by external charge q)