problem solved!!!
#ifdef xSCREEN_3.2
#include "screen3_2.h"
Screen3_2 myScreen;
#elseif defined(xSCREEN_5_0)
#include "screen5_0.h"
Screen5_0 myScreen;
#else
#include "screen3_2.h"
Screen3_2 myScreen;
#endif
it works as expected!! 
small question, this compiles both classes or only one?
thank you so much for your answers, really apreaciate