What size water softener for 1 person at 3 grains per gallon?

It depends entirely on your hardness, which is why household size alone never answers it. At 3 grains per gallon, 1 person uses about 80 gallons a day and remove 240 grains of hardness, which points at a 24,000 grain nameplate regenerating roughly weekly. At half that hardness the same household would be comfortable a size down.

3 grains per gallon is 51 milligrams per liter, which the United States Geological Survey classifies as slightly hard. There is no EPA limit for hardness of any kind, because hardness is not a health contaminant: it has no maximum contaminant level and no secondary standard. It is a scale, appliance life and laundry question rather than a safety one.

At an efficient 8 pounds of salt per cubic foot, a 24,000 grain unit serving this household uses roughly 32 pounds a year. Running the same unit at a maximum 15 pound dose buys more capacity per regeneration but costs considerably more salt per grain removed, which is the trade off the efficiency certification is about.

A metered softener regenerates overnight, so it has to still have capacity left between deciding to regenerate and actually doing it. One day of household use is the usual reserve, which is 240 grains here. Sizing calculations that ignore the reserve overstate the interval and are the reason some households get hard water for an hour every week without understanding why.

Not per grain removed. A larger resin bed regenerates less often and can run at a more efficient salt dose, so the annual salt cost is frequently lower rather than higher. The real limits on oversizing are purchase price, the physical space, and the fact that a bed which sits loaded for more than about two weeks starts to channel, which is why manufacturers force a cycle on a calendar.