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 such decays in terms of the well-known vector- and scalar- $K\pi$ form factors as well as those 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 K*(892) resonance, while the scalar part needs some additional adjustments. Once fixed, one can predict $D_s \to \pi^- K^+ K^+$ decays, finding an excellent agreement with the experimental results.