I think that your birds could have a respiratory infection. I would personally isolate the sick birds. It could be a disease like Chronic Respiratory Disease, or Coryza. It is hard to tell the exact respiratory disease, however (and that's assuming that it is one- it could be caused by dust getting in the eye, or could be an eye infection).
If it's an eye infection or a respiratory infection, it would be a good idea to start antibiotics. Tylan50 is a good antibiotic for respiratory disease. Terramycin eye ointment could help if it is an eye infection. Even Penicillin could help an eye infection.
If it's a respiratory disease, keep in mind that your birds will likely always carry it, and could show signs again in times of stress. An eye infection should eventually clear up.