  We present a model description of the bound $^{17}$B isotope in terms of a
$^{17}$B-n-n three-body system where the two-body subsystems $^{17}$B-n and n-n
are unbound (virtual) states close to the unitary limit. The $^{17}$B ground
state is well described in terms of two-body potentials only, and two low-lying
resonances are predicted. Their eventual link with the Efimov physics is
discussed. This model can be naturally used to describe the recently discovered
resonant states in $^{20,21}$B.
