This question is about the final burn in the landing sequence of the Falcon 9 first stage return to Earth tests.
If I understand correctly, there is usually far more thrust than there is weight at this point. If the maneuver took place a little higher up, the first stage would simply reach zero vertical velocity and immediately start accelerating back up again until it became unstable (see this answer and all the associated comments) or ran out of fuel.
An abort and re-try could be made more feasible if two or only one engine were used once this phase began. Three engines might send the stage too high too fast. Shutting down two engines and doing this with a single engine would improve the possibility that this emergency procedure might work.
However any further discussion of that belongs with this question. Here, I'm asking specifically: Can Falcon 9 first stage landing final burn using one, two, and three engines and what are the Tradeoffs?
The following two quotes are from a list of Falcon 9 first stage landing attempts - these two contain some information about the number of engines used, but it's not enough to make a definitive list of the number of engines used in the final burns of each landing.
Flight 24: Pursuing their experiments to test the limits of the flight envelope, SpaceX opted for a shorter landing burn with three engines instead of the single-engine burns seen in earlier attempts; this approach consumes less fuel by leaving the stage in free fall as long as possible and decelerating more sharply, thereby minimizing the amount of energy expended to counter gravity.
Flight 26: The landing failed in its final moments due to low thrust on one of the first stage engines, caused by the exhaustion of its liquid oxygen fuel supply. That caused the engines to shut down early while the first stage was just above the drone's deck, causing a landing failure.