/*pop3 solve three level atom without radiative transfer */
double pop3(double g1, 
  double g2, 
  double g3, 
  double o12, 
  double o13, 
  double o23, 
  double a21, 
  double a31, 
  double a32, 
  double ex12, 
  double ex23, 
  float *pop2, 
  double abund, 
  double gam2);