Why does my dental crown hurt nearly 3 years later? Sensitive to pressure and temperature changes.

After that much time has elapsed,  you either need a root canal or you have a crack and will likely need an extraction.  I would see a dentist asap to have it evaluated and the associated treatment performed.  Best of luck.  

If you’re
having constant pain without chewing, it usually is a sign that you need to have
a root canal treatment. It’s also possible that the tooth is cracked. Normally,the
teeth should not be sensitive after crown placement, and you should be able to
chew without experiencing any pain. I recommend visiting your dentist to check
for an underlying issue causing the pain