  The information on the symmetry energy and its density dependence is deduced
by comparing the available data on the electric dipole polarizability
$\alpha_D$ of ${}^{68}$Ni, ${}^{120}$Sn, and ${}^{208}$Pb with the predictions
of the Random Phase Approximation, using a representative set of nuclear energy
density functionals. The calculated values of $\alpha_D$ are used to validate
different correlations involving $\alpha_D$, the symmetry energy at the
saturation density $J$, the corresponding slope parameter $L$ and the neutron
skin thickness $\Delta r_{\!np}$, as suggested by the Droplet Model. A subset
of models that reproduce simultaneously the measured polarizabilities in
${}^{68}$Ni, ${}^{120}$Sn, and ${}^{208}$Pb are employed to predict the values
of the symmetry energy parameters at saturation density and $\Delta r_{\!np}$.
The resulting intervals are: $J\!=\!30 \text{-}35$ MeV, $L\!=\!20 \text{-} 66$
MeV; and the values for $\Delta r_{\!np}$ in ${}^{68}$Ni, ${}^{120}$Sn, and
${}^{208}$Pb are in the ranges: 0.15\text{-}0.19 fm, 0.12\text{-}0.16 fm, and
0.13\text{-}0.19 fm, respectively. The strong correlation between the electric
dipole polarizabilities of two nuclei is instrumental to predict the values of
electric dipole polarizabilities in other nuclei.
