Plutonium-242 has a particularly low cross section for thermal neutron capture; and it takes three neutron absorptions to become another fissile isotope (either curium-245 or plutonium-241) and then one more neutron to undergo fission.
Plutonium-242's unique characteristic of having a low cross section for thermal neutron absorption makes it necessary for a sequence of events involving three neutron absorptions to transform into another fissile isotope, which then requires an additional neutron for fission to take place.