Your status entitles you to 70lbs per bag instead of the 50lbs per bag allowance given to non-elites. But the number of pieces is controlled by which cabin you're flying in, not your status. It's two pieces for coach, three for FC/WBC.

This seems like a silly policy, since EUA isn't exactly 100% predictable. But I'm pretty sure it's been the policy for at least a couple years.

You must have just gotten generous ticket agents in the past who checked your third bag for free. Or maybe you were flying with more than one passenger on the same PNR?