3 days to allow emergency response to arrive is a lot of buffer, when otherwise after 2 days you have to start moving to find water and plan to find food.

Combined with things in people's houses that likely gets you to about a week before anyone is going without.

Mormons also (are encouraged to) store water and other basic necessities like toilet paper. It's not unusual to see a couple 55-gallon drums of water in a basement or garage of a Mormon house. They've also heavily adopted the filtering straws/pumps for areas where there are natural sources of water