a charge of -3Q is transferred to a solid metal sphere of radius r.where will this excess charge resides?

Consider a gaussian surface infinitesimally inside the surface of the sphere. Because the sphere is a conductor, the electric field throughout the condictor is zero implying that the flux through the gaussian surface is zero. Therefore, according to Gauss' law there is zero charge within the sphere. Thus the entire excess charge will reside on the surface of the sphere.