All three of those are required auxiliary processes that MC uses as part of its multithreading model. Each performs a variety of services for the application. For example, you're right that jrweb.exe provides some services for the built in browsers, but it is also used for general parts of the UI and for things like
automatic metadata lookup.
If your security suite restricts any of them, or the other processes MC uses (such as "core" Media Center 21.exe executable, the LAV and possibly madvr DLLs used for playback, or the MC21.exe command line launcher applet), then it will fail to function properly.
There is no comprehensive list of each of their functions, if that's what you're looking for.
