  We study the discovery prospect of different three body lepton number
violating~(LNV) meson decays $M_{1}^{-}\to\ell_{1}^{-}\ell_{2}^{-}M_{2}^{+}$ in
the framework of right handed~(RH) neutrino extended Standard Model~(SM). We
consider a number of ongoing experiments, such as, NA62 and LHCb at CERN, Belle
II at SuperKEK, as well as at the proposed future experiments, SHiP, MATHUSLA
and FCC-ee. The RH Majorana neutrino $N$ mediating these meson decays provides
a resonant enhancement of the rates, if the mass of $N$ lies in the range
$(100\, \text{MeV}-6\, \text{GeV})$. We consider the effect of parent mesons
velocity, as well as, the effect of finite detector size. Using the expected
upper limits on the number of events for the LNV decay modes, $M_{1}^{-}
\to\ell_1^{-}\ell_2^{-}\pi^{+}$~($M_{1}=B, B_c,D, D_{s}\,\text{and}\,K$), we
analyze the sensitivity reach of the mixing angles $|V_{e N}|^{2}$, $|V_{\mu
N}|^{2}$, $|V_{\tau N}|^{2}$, $|V_{e N}V_{\mu N}|$, $|V_{e N}V_{\tau N}|$ and
$|V_{\mu N}V_{\tau N}|$ as a function of heavy neutrino mass $M_{N}$. We show
that, inclusion of parent meson velocity can account to a large difference for
active-sterile mixing, specially for $D$, $D_s$ meson decay at SHiP and $K$
meson decay at NA62. Taking into account the velocity of the $D_s$ meson, the
future beam dump experiment SHiP can probe $|V_{eN}|^2 \sim 10^{-9}$. For RH
neutrino mass in between 2 - 5 GeV, MATHUSLA can provide best sensitivity reach
of active-sterile mixings.
