If you don't give your new tabs an extension, they will be concatenated together with the main tab to create a single file (and then preprocessed to generate function prototypes), so you don't need #include's. If you add .c, .cpp, or .h tabs to the sketch, you'll want to #include the appropriate headers as normal for a non-Arduino program. The .c and .cpp files will be automatically linked against your main tab, though. Also, note that the main tab is compiled as C++, so if you add .c files / tabs to the sketch, you'll need to wrap the function declarations in an extern "C" {} block (conditional on #ifdef __cplusplus), as you would to use C code from C++ code in any other context.