The molecularity is the number of molecules that react in an elementary process. Unimolecular steps are not as common as Bimolecular steps. Radioisotopes decay via a unimolecular elementary step. Bimolecular steps involve the collision of two reacting molecules. This is the most common elementary step.
Termolecular steps involve the simultaneous collision of three reacting species.
This is a very rare occurance. It is important to distinguish between molecularity and order. In contrast, the order is the sum of the exponents in the rate law for the overall reaction. It can only be determined experimentally and can be 0, fractional or integral. The challenge of Kinetics is to determine the elementary steps which lead to the experimentally observed rate law and give the proper overall stoichiometry.