  Within the hyperspherical harmonics approach the three-body problem is
reduced to a motion of one effective particle in a "strongly deformed" field,
which is described in coupled-channel formalism. This method is especially
suited to studies of phenomena characterized by genuine three-body dynamics,
e.g. Borromean haloes and true three-body decays. The reduction of the
hyperspherical equations set to a single-channel Schr\"odinger equation
provides the basis for the use of the standard quasiclassical expression for
calculations of widths for true three-body decays. We demonstrate that the
quasiclassical approach by itself is quite precise in application to typical
profiles of the three-body effective potentials. However, the reduction to
single-channel formalism leads to significant overestimation of the two-proton
width $\Gamma_{2p}$. This is demonstrated by the example of the $^{17}$Ne first
excited $3/2^-$ state decay, questioning, however, the applicability of such an
approximation in general.

Nuclear Theory