I find that when back filling with concrete and the round posts, I get "good enough" without any tripod to stabilize. "Good enough" is about all you can get with these posts considering they're somewhat irregular and often have a slight bend...
I use dirt for all wooden fences. (I prefer dowelled fencing for horses, but cows are rough on them without a hot wire). Dirt makes it easier to replace a post if something goes wrong (even pressure treated rots). If I ever had enough sand, that might be better than dirt. Gravel might be better for drainage, but I have never done it because I am concerned the post might start leaning If an animal rubs against it (that is just supposition though). I do have some 6" pressure treated rounds that I use to change direction on a barbed wire fence that I backfilled with concrete to keep the pole from leaning from the pull of the fence.
Wouldn't it be easier to just get a bigger auger? It'll handle the rocks easier.
I have been there. Spent a lot of time hooking it up, getting into position to start, discovering I was not really in position to start, starting, not making progress, exiting the holes, using a pry bar to chip away at the 'too hard' dirt and rocks, trying other locations where the dirt might be easier to dig in, and then abandoning it all and digging my holes by hand.