Most people with a house have a hot water heater, so they have 30 to 50 gallons of water on hand. If you have a neighbor with a pool, you have water for cleaning things, and flushing a toilet if your plumbing is in tack. It is harder for those in an apartment.

Predictions are that SoCal would have little water for three weeks, not three days. After a trial several years ago, I decided that 3 gal of water per person per day was a better estimate of what would be required. I then kept one 55 gallon plastic container per person, plus some smaller containers for water transfer. My wife always complained, until the day we lost water due to a water main break.