Highlight the contents of your "B.h" file and copy it to your clipboard (Ctrl+C). Go to A.h and highlight the line '#include "B.h"' and hit paste (Ctrl+V). This is literally what the compiler does when processing the "#include" pre-processor command while creating .o files. This is why source files (*.cpp) have corresponding object files but header files do not.
Also you'll want to watch out for something called "Circular Dependency", it's a real PITA to prevent when you start doing stuff like this. It's often better\easier to just have all of your classes inherit from a common base class then it is to have one class contain instances of the other two.