You should only learn all three languages if you want to know all three languages.

Learning C as a stepping stone to C++ is a waste of time. You might think it's a good idea, because C++ is based on C, and most of the features of C are available in C++. But writing good C code requires idioms that are useless in C++.

For instance, suppose you need to write a function that opens a file, reads the contents into a some data structure, and then returns a result. Before the function returns, it needs to free the memory in the data structure and close the file. If you write this in C, you either have to make sure you only return at the end, or use nested "if" statements, or use "goto" statements. But in C++, you don't have to worry about this, because the object destructors will handle it.