  The chiral symmetry of QCD requires energy-dependent pionic strong
interactions at low energies. This constraint, however, is not fulfilled by the
usual Breit--Wigner parameterization of pionic resonances, leading to masses
larger than the real ones. We derive relations between nonleptonic three-body
decays of the $B$-meson into a $D$-meson and a pair of light pseudoscalar
mesons based on SU(3) chiral symmetry. Employing effective field theory
methods, we demonstrate that taking into account the final-state interactions,
the experimental data of the decays $B^-\to D^+\pi^-\pi^-$, $B_s^0\to
\bar{D}^0K^-\pi^+$, $B^0\to\bar{D}^0\pi^-\pi^+$, $B^-\to D^+\pi^-K^-$ and
$B^0\to\bar{D}^0\pi^-K^+$ can all be described by the nonperturbative
$\pi/\eta/K$-$D/D_s$ scattering amplitudes previously obtained from a
combination of chiral effective field theory and lattice QCD calculations. The
results provide a strong support of the scenario that the broad scalar charmed
meson $D^\ast_0(2400)$ should be replaced by two states, the lower one of which
has a mass of around 2.1 GeV, much smaller than that extracted from
experimental data using a Breit--Wigner parameterization.
