An elementary termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. Termolecular elementary reactions are uncommon because the probability of three particles colliding simultaneously is less than one one-thousandth of the probability of two particles colliding. There are, however, a few established termolecular elementary reactions. The reaction of nitric oxide with oxygen appears to involve termolecular steps:
$$2NO+O_2⟶2NO_2$$
$$rate=k[NO]^2[O_2]$$
Likewise, the reaction of nitric oxide with chlorine appears to involve termolecular steps:
$$2NO+Cl_2⟶2NOCl$$
$$rate=k[NO]^2[Cl_2]$$