The Cabibbo allowed modes $D\to\bar{K}\pi\pi$ have proven
important for hadronic physics (evidence for the exotic $\kappa$
meson) and for precision testing of the Standard Model (measurement of
the CKM parameter $\gamma$). We develop a general theoretical
description of such modes combining isospin symmetry (which is
particularly predictive here) and a dispersive reconstruction theorem,
from which we derive a set of Khuri-Treiman type equations which
enable one to take three-body rescattering effects into account. These
could be more significant in D than in K or B decays and we look for
specific signatures. A first test of this approach is presented using
experimental results on the $D^+\to K_S\pi^0\pi^+$ and the $D^0\to
K_S\pi^-\pi^+$ modes.