Sounds exactly like my chicks.
They were only 7 weeks old drumcrow, so maybe that's why they didn't do so well. They went downhill very quickly, & despite antibiotics were dead within a day/36 hours from the first sneeze.
The 3rd chick totally escaped the illness, no symptoms at all, but have dosed her with Baytril for a few days just to be safe.
But don't read that as all "doom & gloom" drumcrow, they were only babies remember, yours will be much stronger
Which antibiotic are they on? Ideally it should be Tylan or Baytril.
Oh BTW, ideally you should separate the infected birds from the others.