Larry,
All three of your shots, I also thought of in my mind, but not knowing 3C I therefore do not know what constitutes playing positioning.
I believe it means; leaving a next shot that is easy. But I am not sure what constitutes an easy shot, other than it seems leaving an ob by a corner gives extra chances of making a hit going in and then often times making the hit coming back out, which adds probability to making the hit.
Or,
Possibly it means leaving any natural 3C shot.
So there are probably a number of shot scenarios that are considered position leaves, and probably those have been very much documented over the years.
One thing that amazes me, and I believe I have this correct, billiards has been well documented for centuries. Shots, scenarios, so there has been a lot of information out there. But is this true, Phelan is the one that put dots on to tables, and that would not be until 1850 or so. There was a french snooker table in Brighman's that dated 18th century I believe it had dots. No one ever played on it for the pockets were incredibly small. There would be no 125 runs on that table.
You guys correct me if I am wrong, thanks! Whitey