Regular crawl spaces are problematic as fallout shelters, IMO. Even with 3 floors, they won't have adequate shielding from gamma without reinforcement. Besides being uncomfortable, they are very hard to seal and then vent at a good level, often damp, hard to get stuff into, hard to create a toilet area, and there's a risk of collapse or getting stuck down there without completely reworking an entry/exit.

I think it is usually easier and safer to convert a garage (almost always directly on a slab), or some other room in the house, or construct a cement shed/shelter than it is to rework a standard crawl space.