  The dynamics of systems of two and three planets, initially placed on
circular and nearly coplanar orbits, is explored in the proximity of their
stability limit. The evolution of a large number of systems is numerically
computed and their dynamical behaviour is investigated with the frequency map
analysis as chaos indicator. Following the guidance of this analysis, it is
found that for two-planet systems the dependence of the Hill limit on the
planet mass, usually made explicit through the Hill's radius parametrization,
does not appear to be fully adequate. In addition, frequent cases of stable
chaos are found in the proximity of the Hill limit. For three-planet systems,
the usual approach adopted in numerical explorations of their stability, where
the planets are initially separated by multiples of the mutual Hill radius,
appears too reducing. A detailed sampling of the parameter space reveals that
systems with more packed inner planets are stable well within previous
estimates of the stability limit. This suggests that a two-dimensional approach
is needed to outline when three-planet systems are dynamically stable.
