Arduino library making
I'm going to assume your library structure is like this because the structure you described above would not cause that error:

AAA
|_src
    |_AAA.h
    |_BBB.h
    |_CCC.h
    |_BBB.cpp
    |_CCC.cpp


To be considered valid by the Arduino IDE, a library must either have a .h file in the root folder or a library.properties file if you are using the 1.5 library format. Putting all the source files in the src subfolder means your library is in the 1.5 library format. So you need to add a library.properties file in the root of the library formatted as described here: https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#libraryproperties-file-format. Then your library should look like this:

AAA
|_library.properties
|_src
    |_AAA.h
    |_BBB.h
    |_CCC.h
    |_BBB.cpp
    |_CCC.cpp