  Motivated by recent work on three-point QCD sum rules in heavy quark physics,
we use the simple quantum mechanical models to study the basic issue of duality
in three-point sum rules. We show that while in all of these models the duality
in two-point sum rules works fine, the duality in three-point sum rules may be
100%- violated, leading to completely unreliable predictions for the matrix
elements in question. The implications for three-point QCD sum rules are
discussed. A new estimate for the parameter $\lambda_1$ of HQET is given.
