[strat61caster]

Accoding to the book in your glovebox the coolant is a 100k mile service interval iirc.
Yes, add distilled water.

It's a tank of water, after 3 years some of it evaporated, not rocket science, top off and it will be fine for another 2+ years.

Edit: And don't fill it to max when cold, the level will fluctuate with temperature, if you put it at max when cold you will over flow when the car is up to temperature, you probably want the level between 50% and low when the car is cold, pop the hood after a drive and see where the coolant is at.