Are there any common elemental (single-step) reactions with three reactants?
Everything I've found either says that they're very rare, or can be broken down into two bimolecular steps with combined reaction rates far greater than the termolecular step.
What reactions require an indivisible termolecular step?
Termolecular reactions almost never happen. The chance of three molecules "colliding" at the same instant is so small as to be ignorable.
On the other hand there are "almost" termolecular reactions when one molecule hits another and "sticks" to it, and then a third molecule hits the two that are stuck together. This most often happens when one of the molecules is part of a solid. But this is really two sequential bimolecular reactions.
