  Matrix elements of weak currents involving light multi-meson states are
important in many hadronic decays of both heavy leptons and heavy mesons. In
this paper we focus on the latter case where the current size of the data set
demands better models. The specific case of three-kaon weak matrix elements is
considered and expressed as a relatively simple structure, which generalizes
naturally the concept of form factor. We propose a model for the decay $D^+ \to
K^+ K^- K^+$ as an alternative to isobar model, with free parameter predicted
by the theory to be fine-tuned by a fit to data. An important qualitative
outcome is that we encompass naturally all final states topologies, which
involve necessarily proper multi-particle structures and cannot be decomposed
into simpler two-body processes. This aspect represents a significant
improvement when compared to isobar model, often employed in analyses of
heavy-meson decay data.
