  Despite the undeniable success of the Standard Model of particle physics
(SM), there are some phenomena that SM cannot explain. These phenomena indicate
that the SM has to be modified. One of the possible ways to extend SM is to
introduce heavy neutral leptons (HNL). To search for HNL in intensity frontier
experiments, one has to consider HNL production both in 2-body and 3-body
decays of some mesons. We verified the possibility to use the PYTHIA approach
for the calculation of HNL production in 3-body decays. We conclude that the
PYTHIA approach is quite suitable for the estimation of the sensitivity region
for the intensity frontier experiments.
