The Lagrangian for three interacting real fields $\phi_1, \phi_2, \phi_3$ is
$$
e=\frac{1}{2}\left(\partial_\mu \phi_i\right)^2-\frac{1}{2} \mu^2 \phi_i^2-\frac{1}{4} \lambda\left(\phi_i^2\right)^2
$$
with $\mu^2<0$ and $\lambda>0$, and where a summation of $\phi_i^2$ over $i$ is implied. Show that it describes a massive field of mass $\sqrt{-2 \mu^2}$ and two massless Goldstone bosons.