In this article, we study the three-pion $B_c$-decays $B_c^+ \to J/\psi\, \pi^+\pi^-\pi^+$, $\eta_c\, \pi^+\pi^-\pi^+ $ with dominance of the intermediate axial-vector meson $a_1(1260)$ and vector meson $\rho(770)$ in the $\pi^+\pi^-\pi^+$ and $\pi^+\pi^-$ invariant mass distributions respectively, and make predictions for the branching fractions and differential decay widths. The ratio between the decays $B_c^+ \to J/\psi\, \pi^+\pi^-\pi^+$ and $B_c^+ \to J/\psi\, \pi^+ $ is compatible with the experimental data within uncertainties, other predictions can be confronted with the experimental data in the future at the LHCb.

Comment: 9 pages, 6 figures, slight revision