Is there any probability that dark matter is either made of 3 neutrons bound together or it can be neutronium?
It is unlikely that dark matter in neutrons.
Neutrons are unstable unless bound to protons in an atomic nucleus or in a neutron star.
Free neutrons decay with a half life of just over 10 minutes into a proton an electron and an electron antineutrino. Three neutrons bound together would not be stable. Two of the neutrons would decay into protons to form Helium 3.
Neutronium is the stuff of neutron stars. These are formed by gravitational collapse of the core of a large star. The reason why these objects are just neutrons is that gravity has overcome the electron degeneracy pressure and the Pauli exclusion principle doesn't allow electrons to be in the same state and neutrons can't decay into protons and electrons.
The most likely candidates for dark matter are particle which haven't yet been detected or black holes.