Using water for three miles wont do any harm, although you should replace it with coolant as soon as you get the chance. The main reason you use coolant is it has a lower freezing temperature so wont crack the block in subzero temperatures, it contains corrosion inhibitors, and has a higher boiling temperature. The cooling system (if sound) is pressurized anyway so even water will boil at a higher temperature than 100c. From the description it sounds like you were overdue a change anyway. Motor factors usually sell two types of coolant, Blue (needs to be changed every 2 years) and orange (every 5 years). You can use either but don't mix the two. If unsure flush the entire system completely with water once or twice and then use one of them.