Learning how to program in any language will make it easier to pick up any other language, because the main burden for a beginner is how to think programmatically. However once you're enough past that wall, being an expert on one language will mostly only help pick up languages that are similar. So if you knew C++, you could pick up the syntax and probably most of the semantics of the others very quickly, because they are similar in that regard. But you'd still probably struggle to actually program in C, because C is lower level (has way fewer features) than C++.

Technically speaking, C is a subset of C++. But that doesn't mean being a good C++ programmer automatically makes you a good C programmer.

C# is similar to the other two in syntax as well, but it's much more like Java than either of them.