A) is your idle stop. It sets the opening of the carb at idle. Clockwise should be higher idle speed. Usually a set it and forget it screw. Once you get a decent idle speed you can leave this alone.
B) is your High speed mixture needle, or HSN. Controls Rich/lean at speed. Clockwise makes it leaner. This needle you will be tweaking all the time. Changing conditions require retuning your engine. Start at whatever settings the manual says, and start going clockwise from there, 1/8th of a turn at a time, keeping an eye on performance, smoke, and temps. Search here at RCU and google for nitro tuning guides.
C) Hard to tell without knowing the engine. If there is another screw on the opposite side of the carb where the rubber boot is, then this is what is called the mid speed needle. Something you should set to what the manual recommends, and leave it there until you have a much greater understanding of tuning. It is a subtle adjustment but messing with it can throw your tuning out of whack if you don't know what you're doing.
If there is no screw opposite C), then it is your Idle mixure, or LSN (low speed needle). It controls the fuel mixture at idle. Needs to be tuned infrequently once set up. Controls how well your car will idle, and how much punch it has off the line. Usually sensitive to changes. Clockwise = leaner. If this is out of whack it can make your engine hard to start. Again, search for a tuning guide to learn how to properly tune your LSN.
When i say set it to what the manual recommends, i mean if you've moved them already. If not, just leave them how they came.
Here is a diagram illustrating the needles' position a little better. Typical 3-needle carb. Top-down view.
