For a sealed can of smokeless powder three years is not old at all. I have powders that are much older that are very safe and show no signs of problems. The vapor trail you see may be more of a sign of ambient conditionsand have nothing to do with powder. But for the $20 or so that a pound cost you and if you're worried about it, pitch it. Me, I'd use it. Or buy another pound of the same powder and do a comparative check....any powder that yields "sub dime groups" would be worthwhile to have around anyway.