  The study of local \cp \ asymmetry in Dalitz plots of charm (& beauty) decays
gives us more information about the underlying dynamics than the ratio between
total rates. In this paper we explore the consequences of the constraint from
\cpt \ symmetry with emphasis on three-body $D$ decays. We show simulations of
$D^{\pm} \to \pi^{\pm}K^+K^-$ and discuss correlations with measured $D^{\pm}
\to \pi^{\pm}\pi^+ \pi^-$. There are important comments about analysies of
recent LHCb data in \cp\ asymmetries for $B^{\pm}$ decays to three-body final
states.
