  Nuclear decays into three-particle channels are considered in a few-body
approach of hyperspherical harmonics with emphasis on simultaneous, or direct,
emission of two protons. General conditions of direct decays are described and
their main features, being experimentally established in decays of light
nuclei, are reported. The analysis method based on an expansion of decay
amplitude into a series of hyperspherical harmonics is reviewed. The basis of
hyperspherical harmonics functions is a generalisation of the spherical
function basis in three-body systems. The method is tested on analysis of the
direct 2p-decay of Be^6 where the three-body components in the nuclear
structure of Be^6 have been studied. In particular, the observed strong
proton-proton correlations are treated as a manifestation of a specific
three-body quantum effect: the kinematic focusing of fragments over momenta and
in space. The hyperspherical harmonics method is applied for the predictions of
proton-proton correlations and life-time estimates of the nuclei Mg^19, Ca^34
and Ni^48 - candidates for two-proton radioactivity. Each direct 2p-decay
should result in a set of peaks in the E_(p-p) spectrum whose number and
positions depend on the structure of initial nucleus, opposite to the diproton
model, predicting the He^2 emission with one peak at E_(p-p)~0 in all cases.
