Those early 3 year replacement cycles are so you'll run out and buy another. If you had the helmet stored then it wasn't exposed to anything that would make the helmet unsafe unless some sort of chemical got spilled on it. Even if you were using the helmet a helmet is good for at least 5 years depending on the individual who is wearing it chemical disposition. But the way you tell a helmet is due for replacement is after a ride and you sweated you may feel like there is fine particles of dirt on your forehead and or hair, and you'll wonder where that came from, it came from the degradation of the Styrofoam and it's time to get a new one, it's still safe to ride for a short time while you shop for a new one. The other clue is if the Styrofoam appears to have a depression where one shouldn't be, time to get another; and one last clue is if there are cracks forming on the Styrofoam. Or did you crashed and smacked the helmet it will probably need to be replaced. And one of those 4 conditions is a clue to get another.
The crashing thing is up to the individual's knowledge about what to look for in the way of damage to the helmet, Styrofoam depression and cracks are a big clue, but sometimes you may not hit all that hard and the helmet could still be good, but if you're not sure better safe than sorry and replace it.
Of course if the helmet you're talking about was stored since the 80's well you may want to buy a new one. Newer helmets made in the late 80's and onward the companies treated the Styrofoam to prevent UV from doing as much damage as they use to get.
Most of my helmets when I lived in California's Mojave Desert area, where it would get very hot and a person sweats a lot, my helmets lasted 6 to 7 years, since I moved away from that area and now live in a more cooler climate my current helmet is 8 years old with no signs of degradation going on. I am on my second set of pads, if the second set of pads go bad before the Styrofoam does then I'll probably replace the helmet.
You can read more about what I said here if you're having difficulty believing what I said:
http://www.bhsi.org/replace.htm
Keep in mind, you have marketing forces at work, and they want you to buy stuff over and over, and helmet plays in the heart strings, so by golly better replace that thing every 3 years so the helmet company can make more money...while you lose more money.