  We compute the effective potential $V_{\rm eff}(\phi)$ for one-component real
scalar field $\phi$ in three Euclidean dimensions (3D) in the case of
spontaneously broken symmetry, from the Monte Carlo simulation of the 3D Ising
model in external field at temperatures approaching the phase transition from
below. We study probability distributions of the order parameter on the
lattices from $30^3$ to $74^3$, at $L/\xi \approx 10$. We find that, in close
analogy with the symmetric case, $\phi^6$ plays an important role: $V_{\rm
eff}(\phi)$ is very well approximated by the sum of $\phi^2$, $\phi^4$ and
$\phi^6$ terms. An unexpected feature is the negative sign of the $\phi^4$
term. As close to the continuum limit as we can get ($\xi \approx 7.2$), we
obtain
  $$ {\cal L}_{\rm eff} \approx
  {1 \over 2} \partial_\mu \phi \partial_\mu \phi +
  1.7 (\phi^2 - \eta^2)^2 (\phi^2 + \eta^2). $$
  We also compute several universal coupling constants and ratios, including
the combination of critical amplitudes $C^- (f_1^-)^{-3} B^{-2}$.
