  In this paper, we study the three-body decays $B^0/B^0_s \to \eta_c f_0(X)\to
\eta_c \pi^+\pi^-$ by employing the perturbative QCD (PQCD) factorization
approach. We evaluate the $S$-wave resonance contributions by using the
two-pion distribution amplitude $\Phi_{\pi\pi}^{\rm S}$. The Breit-Wigner
formula for the $f_0(500)$, $f_0(1500)$, and $f_0(1790)$ resonances and the
Flatt\'e model for the $f_0(980)$ resonance are adopted to parameterize the
time-like scalar form factors $F_{s}(\omega^2)$. We also use the D.~V.~Bugg
model to parameterize the $f_0(500)$ and compare the relevant theoretical
predictions from different models. We found the following results: (a) the PQCD
predictions for the branching ratios are ${\cal B}(B^0\to \eta_c
f_0(500)[\pi^+\pi^-])= \left ( 1.53 ^{+0.76}_{-0.35} \right ) \times 10^{-6}$
for Breit-Wigner model and ${\cal B}(B^0\to \eta_c f_0(500)[\pi^+\pi^-])= \left
( 2.31 ^{+0.96}_{-0.48} \right ) \times 10^{-6}$ for D.~V.~Bugg model; (b) $
{\cal B}(B_s\to \eta_c f_0(X)[\pi^+\pi^-] ) =\left ( 5.02^{+1.49}_{-1.08}
\right )\times 10^{-5}$ when the contributions from
$f_0(X)=(f_0(980),f_0(1500),f_0(1790))$ are all taken into account; and (c) The
considered decays could be measured at the ongoing LHCb experiment,
consequently, the formalism of two-hadron distribution amplitudes could also be
tested by such experiments.
