  A possibility of formation of the three reaction products having comparable
masses at the spontaneous fission of $^{252}$Cf is theoretically explored. This
work is aimed to study the mechanism leading to observation of the reaction
products with masses $M_1=$136---140 and $M_2=$68---72 in coincidence by the
FOBOS group in JINR. The same type of ternary fission decay has been observed
in the reaction $^{235}$U(n$_{\rm th}$,fff). The potential energy surface for
the ternary system forming a collinear nuclear chain is calculated for the wide
range of mass and charge numbers of constituent nuclei. The results of the PES
for the tripartition of $^{252}$Cf(sf,fff) shows, that we have favorable
dynamical conditions for the formation of fragments with mass combinations of
clusters $^{68-70}$Ni with $^{130-132}$Sn and with missing cluster
$^{48-52}$Ca.
