  We develop a model to reproduce the mass distributions of pairs of mesons in
the Cabibbo-suppressed $D^+_s \to K^+ \pi^+ \pi^-$ decay. The largest
contributions to the process comes from the $D^+_s \to K^+ \rho^0$ and $D^+_s
\to K^{*0} \pi^+$ decay modes, but the $D^+_s \to K^*_0(1430) \pi^+$ and $D^+_s
\to K^+ f_0(1370)$ modes also play a moderate role and all of them are
introduced empirically. Instead, the contribution of the $f_0(500)$, $f_0(980)$
and $K^*_0(700)$ resonances is introduced dynamically by looking at the decay
modes at the quark level, hadronizing $q \bar{q}$ pairs to give two mesons, and
allowing these mesons to interact to finally produce the $K^+ \pi^+ \pi^-$
final state. These last three modes are correlated by means of only one
parameter. We obtain a fair reproduction of the experimental data for the three
mass distributions as well as the relative weight of the three light scalar
mesons, which we see as further support for the nature of these states as
dynamically generated from the interaction of pseudoscalar mesons.
