No soundstage has actually held three houses at once. There closest we've gotten to it was in the 90s when they designed a triple-run house (a house with three unique paths) for soundstage 22. It would be extremely difficult to fit three actual houses in a soundstage with the way houses are designed now. Facades are so big, one, and two, the entry/exit locations are locked in place.
So... here is a general mock-up of how Carnival Graveyard and Scary Tales fit in their soundstage. The actual houses aren't drawn out, and it's not officially to scare, but it gives a good idea. Also, houses don't go right against the soundstage walls. There's a 3-5 foot buffer for emergency exits.
