In this work, we perform a combined study of $D^+ \to K^-\pi^+\pi^+$ and $D_s \to \pi^- K^+ K^+$ decays using the naive factorization approach.
The formalism allows for a description of both decays in terms of the well-known vector- and scalar- $K\pi$ form factors, together with a form factor appearing in semileptonic $D^{+}\to K^-\pi^+\ell^+\nu$ decays. We propose a useful---yet simple---parametrization to describe the latter that incorporates unitarity and analyticity constraints. As a result, we find a satisfactory description for the $P$-wave part in $D^+ \to K^-\pi^+\pi^+$ decays, dominated by the $\bar{K}^*(892)^0$ resonance, while the full description requires minor adjustments for the $S$-wave.
The final description allows to predict the $D_s \to \pi^- K^+ K^+$ decay, that is in nice agreement with data and provides an excellent consistency check of our study.