Make sure line1
is defined when nothing else is:
#if !defined(line1) && !defined(line2) && !defined(line3)
#define line1
#endif
Generate an error if more than one line is defined:
#if (defined(line1) && defined(line2)) || (defined(line1) && defined(line3)) || (defined(line2) && defined(line3))
#error "Your message here"
#endif