Why are termolecular steps infrequently seen in chemical reactions? the probability of the simultaneous coll?

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