In vanilla xcom1, none of the UFO trajectories use mission zone 3 (see ufoTrajectories.rul)... so the game wouldn't crash even if they were empty; they are just not used for UFO trajectories.
As for why you never get a terror mission there... well, because the regions, which don't have any valid areas in their spawnZones are skipped.
Or in simpler words, the game checks whether a terror mission can spawn there or not... and if not, it just tries somewhere else.