The question that isn't being asked is if you have only bag, does it have to be the 50 lb weight or can it be the "extra" at 70 lbs

I'd be amazed if any UA TA, at this point, would let the >50 lb bag slide (1 or 3) without an argument over additional fees with a domestic itinerary. I think it's a typo on the website (concept, I know) and all domestic limits are 50 lbs. As a *G, you do get to check the 3rd bag. That's my opinion and is based on nothing factual whatsoever.