  The ternary cluster decay of heavy nuclei has been observed in several
experiments with binary coincidences between two fragments using detector
telescopes (the FOBOS-detectors, JINR, Dubna) placed on the opposite sides from
the source of fissioning nuclei. The binary coincidences at a relative angle of
180$^0$ deg. correspond to binary fission or to the decay into three cluster
fragments by registration of two nuclei with different masses
(e.g.$^{132}$Sn,$^{52-48}$Ca,$^{68-72}$Ni). This marks a new step in the
physics of fission-phenomena of heavy nuclei. These experimental results for
the collinear cluster tripartition (CCT), refer to the decay into three
clusters of comparable masses. In the present work we discuss the various
aspects of this ternary fission (FFF) mode. The question of collinearity is
analysed on the basis of recent publications. Further insight into the possible
decay modes is obtained by the discussion of the path towards larger
deformation, towards hyper-deformation and by inspecting details of the
potential energy surfaces (PES). In the path towards the extremely deformed
states leading to ternary fission, the concept of deformed shells is most
important. At the scission configuration the phase space determined by the
PES's leads to the final mass distributions. The possibility of formation of
fragments of almost equal size ($Z_i$ = 32, 34, 32, for $Z$=98) and the
observation of several other fission modes in the same system can be predicted
by the PES. The PES's show pronounced minima and valleys, namely for several
mass/charge combinations of ternary fragments, which correspond to a variety of
collinear ternary fission (multi-modal) decays. The case of the decay of
$^{252}$Cf(sf,fff) turns out to be unique due to the presence of deformed
shells in the total system and of closed shells in all three nuclei in the
decay.
