Three concentric metallic spherical shells of radii $\mathrm{R}, 2 \mathrm{R}, 3 \mathrm{R}$, are given charges $\mathrm{Q}_1, \mathrm{Q}_2, \mathrm{Q}_3$, respectively. It is found that the surface charge densities on the outer surfaces of the shells are equal. Then, the ratio of the charges given to the shells, $\mathrm{Q}_1: \mathrm{Q}_2: \mathrm{Q}_3$, is
