$${\frac{\rho_1}{\rho_2}=\frac{\Delta U_1l_2}{\Delta U_2l_1}, \quad \quad \frac{\rho_1}{\rho_3}=\frac{\Delta U_1l_3}{\Delta U_3l_1}, \quad \quad \frac{\rho_2}{\rho_3}=\frac{\Delta U_2l_3}{\Delta U_3l_2}}$$