In all of the examples (a, b, and c) in the question, the use of "please" is in my view quite natural, and would be understood, and not thought odd, by a fluent speaker.
In each of these "please" could be omitted without changing the meaning, although the statement might be thought less polite or more peremptory. That depends on the relationship of the speakers, and of the level of formality of the situation. It also depends on teh tone of voice used.
The word "please" is often used with a request, and far less often with an inquiry. It was once common to use "please" at the start of an inquiry when a lower status person is addressing a higher status one, or when one is addressing a stranger. This might be thought of as a request for the attention for the person addressed. Some examples:
- Please, Ma'am, I don't know what to do next.
- Please, can you tell me the way to the station?
The first of these is now thought rather formal and old-fashioned.
"Please" can also be used at the end of a request for information. For example
- Would you tell me how to get to the market, please?
- What time is it, please?
- How much does that cost, please?
One could think of the "please" in each of these cases as a request for the person's attention, but any question could be thought of in that way. I think of it as merely a more polite, and slightly more formal, form of inquiry. There are very few inquiries in which "please" would actually be incorrect, although it is often less common than it once was.
The word "please" can also be used for emphasis, as in :
Please be careful.
It can also be used in an ironic sense. For example:
Would you please be careful to check your work this time?
In form this is a request, but in fact it is a polite order, and probably an implied reprimand as well.