3 gallons a day? I genuinely cannot imagine drinking that much. The guys urethra must be made of elastic. (wow, okay, sorry. There's an image I didn't need in my head. Nevermind.)
I shoot for a gallon a day, minimum. Anything on top of that is gravy (so to speak). Even if I were dieting specifically, I'd probably only shoot for an extra half gallon or so. I just get uncomfortable pushing down too much water. In general though, more water is always more goodererer, um, better.
It's generally safe to base your water intake on your urine. If it's yellowish, whether you take a multivitamin or not, you need more water. Also, if you're not draining El Lizardo Grande once an hour, you need more water.