Basement Hallway - Hospital B3 (Resident Evil 3: Nemesis)

On every play-through the game, there will be an encounter with zombies when the elevator stops on one of the three accessible floors of the hospital, thus making it a 33% chance that you will encounter zombies on this floor.

Appear after acquiring the Vaccine Medium from the Experiment Room, but only if you did not encounter zombies when exiting the elevator on this floor, and only you have already acquired the Vaccine Base from the fourth floor.