A termolecular process in a mechanism requires three species to collide at the same time... this is vastly less likely than a bimolecular (two species) collision. Supposedly termolecular processes are actually likely to have bimolecular elementary steps.
For example A + B + C → X + Y
is much more likely to occur as something like
A + B → AB
then
AB + C → X + Y
However, there are some truly termolecular elementary steps. The most common of these involve the third species being a collision partner which takes away excess energy, such as:
A + B + M → AB + M*
where M* is an energetically excited-state form of M
