I too am putting up a fence in about the same soil conditions that you are. I have very rocky soil, mostly softball sized rocks. I was going to attempt to rent a 3pt auger, but my neighbor brought home a trailer version auger from work for me to try out. It pulls behind your truck, is gas powered, has a platform to stand on while digging, a large hand wheel to raise and lower the auger, and a motorcycle style throtle control connected to a brake lever. If he hadn't brought it home, I would have rented one. They rent for about $55 a day.
I used a 9in and a 12in auger, going anywere from 2-4 ft deep. Even with the auger connected to the truck it still shook the heck out of me. It would grab and bind the rocks against the side of the holes. Without the truck I think it would have windmilled the whole trailer. I have never used a 3pt auger so I can't really compare the two, but I think in these conditions the 3pt system wouldn't have been rigid enough to stand up to the abuse.