How much gallons of water do 3 people need for 3 days in case of an earthquake?

This question has many factors. The average amount of water needed to keep a person from dehydration per day is 1 gallon. This is increased under physical strain or heat. Add 2 more gallons a day for bathing. If you wanted to play it safe I would stock pile 3 gallons a day per person of potable water. For 3 people that would be 27 gallons of water for a 3 day period. Another option would be keeping a water purifier or learning how to make potable water using boiling, bleach or other types of purification. This would reduce storage space and would make your water options more travel worthy.