If energy is absorbed, the binding energy increases. The binding energy of these three isotopes is so low that when a neutron is captured, the energy released by rearrangement exceeds it. The nucleus is then no longer stable and must either shed the excess energy, or split into two pieces. By contrast, when the abundant isotope uranium-238 captures a neutron it still has a binding energy deficit of 1 MeV after internal rearrangement. If it captures a neutron with a kinetic energy exceeding 1 MeV, then this energy plus the energy released by rearrangement can over come the binding energy and cause fission.
Heavier elements require relatively more neutrons to stabilize the nucleus.
These neutrons are shed very rapidly by the excited fragments. More neutrons are produced on average than are consumed in fission. Fission is a statistical process. The nucleus rarely splits into pieces with nearly the same mass and atomic number. All of the kinetic energy is released to the environment instantly, as are most of the instantaneous gamma rays.