Theoretical issues with $\Delta S$ in three-body B decays

Theoretical uncertainties with the time-dependent CP asymmetries $\Delta S$ and $A_{\rm CP}$ in $B^0\to K^+K^-K_S$, $K_SK_SK_S$ and $K_S\pi^0\pi^0$ decays are discussed. In order to have a reliable estimate of CP asymmetries, it is very crucial to understand the underlying mechanism for the nonresonant background which dominates the KKK modes.