smell the rock, if it has a very distinct odor, then that means there has been die off. Now if there is nothing in your tank, i would just add it int here cause theres nothing it can really kill. otherwise, you may have to recure it if you have live stock and it smells bad. When it's cured it doesn't smell like much of anything.
Now regarding rock thats been out for 3 years. Basically its no longer live rock, its just ...rock :lol: Now of course if you put it into a tank and after time, it will re-seed itself and become "live rock" once again.