  From a sample of 848 $\pm$ 44 $D_s^+ \to \pi^- \pi^+ \pi^+$ decays, we find
$\Gamma(D_s^+ \to \pi^- \pi^+ \pi^+) / \Gamma(D_s^+ \to \phi \pi^+) =
  0.245 \pm 0.028^{+0.019}_{-0.012} $. Using a Dalitz plot analysis of this
three body decay, we find significant contributions from the channels
$\rho^0(770)\pi^+$, $\rho^0(1450)\pi^+$, $f_0(980)\pi^+$, $f_2(1270)\pi^+$, and
$f_0(1370)\pi^+$. We present also the values obtained for masses and widths of
the resonances $f_0(980)$ and $f_0(1370)$.
