Water is always the real kicker when it comes to being prepared for anything long-term. A gallon per person per day is probably more than you'd really need for drinking purposes unless you live somewhere very warm, and for washing yourselves you don't need food-grade water.
I've often thought about the practicalities of putting some form of basic filter into place so that, for example, rainwater gets stored and used for bathing. Then the runoff from having a bath/shower gets put through some form of basic filter and stored for flushing the toilet. If it's practical to perform #1 and #2 bodily functions in separate places, after flushing a #1 the water can still be used to water the garden.
The problem with storing clean drinking water is twofold. Firstly it needs to stay drinking quality for potentially a long time - since you never know when you'll need it or for how long you've always got periodic windows where you're vulnerable to losing water pressure. If you just put water in a container and leave it there, will it still be good to drink when you need it? And if you periodically drain and refill it, did you make any provision for losing water supply while you're refilling it?
The other issue with storing water is that it requires the assumption you won't have to leave home. Dried food is relatively lightweight - an able-bodied person can carry several days' worth with minimal effort. Water is another issue - a US gallon of water weighs approximately 8lb, so if you're trying to carry three gallons of water per person (enough for only three days, using your projections) then everybody has to haul 24lb of water as well as whatever else they need.
For those thinking in terms of preparing for several weeks or months, having to leave your prepared base blows everything clean out of the water. If you're thinking in terms of a family of 4 using three gallons per day (allowing that children use less, and rounding down a little) then that's 21 gallons per week, ~90 gallons per month. If you need to ship out then being prepared for just one month means hauling ~720lb of water wherever you go, all the while protecting it from the less prepared who would very much like to save you the trouble of hauling it any further.