I would not think you could make money digging post holes with a 3pt digger. I have dug probably in excess of 1000 holes. I hate 3 point hole diggers. To get real production you need one on a loader. I have one on a skid steer and it is so easy to position and it drills fast with the down force. If the locations are marked in advance I could probably drill 200+holes a day with the skid steer. With a 3 point digger in my area I have spent 30 minutes per hole.
You biggest problem is going to be people wanting 4 holes or something like that. On a big job the per hole works good if you can drill them quickly but most of the time you are going to spend more time hooking up your digger, loading tractor, driving there, unloading, loading, driving back, unloading, unhooking than you will actually digging. I don't do any work like that for other people anymore but if I did I would not even show up to do any thing for less than $250.