I was about to start a thread about this yesterday as I am considering logistics as I may move and the 3 X 70lbs will help a lot. There is conflicting information between these two pages:

http://www.united.com/page/article/0,6722,52481,00.html

and http://www.united.com/page/article/0,6722,52482,00.html

It has been a year or two since I have checked any baggage, but I remembered the airport kiosk asking how many to check (1,2,or 3) a couple years ago as 1P. Given the conflicting information I wasn't totally sure for US domestic but it appears 3 X 70lbs is the real deal