My guess is that since I couldn't possibly fill the dishwasher before using it, it would be using 3 gallons for one person's dishes per meal, and I doubt I use a whole gallon while hand washing.
Do you own very few dishes or something? That's the only reason I can imagine not being able to fill it before needing to run it. Even before having a kid my wife and I could easily fill the dishwasher daily since we cook quite a bit.