my US Air miles expired. The reason I was given by a US Air Agent was it was due to 3 years on no activity.
DM policy changed a long time ago. T&C are pretty simple and straight forward. One has 18 months to either fly BIS, purchase something from a DM vendor, or to use the US Air Barclay credit card for one single charged item to earn FF miles and keep an account active for another 18 months. You say it's been three years so you have missed two inactivity cycles.
When one opens a DM account with US and sets their customer preferences to receive e-mail notifications, US will begin to remind you that your clock is ticking down some 3 to 6 months prior to the expiry mark. I suspect that they will also send mail announcing the same. If the ( now expired ) miles in your account are important, you might ask if you can reinstate them for a fee, but given your 3 year lapse, I think the ball game might be over.
Barry