I agree with
@oldhenlikesdogs on all counts.
Don't feed layer feed, use chick starter.
You mention in your opening post this chick is much larger than the others. I do believe you have a meat bird, probably Cornish Cross. Unfortunately, different breeds of chicks can get mixed together at feed stores and employees may not know/notice especially if chicks are somewhat similar in color.
If this chick has a light on 24/7, it will eat 24/7 - this is what they are designed/bred to do, so the full/stuffed crop would be normal. Some people do have success in keeping them alive past butchering age, but monitor/limit their feed intake from the beginning. Meat birds are also prone to have difficulty with their legs since they gain weight so quickly and sometimes they can have heart/organ failure as well (all due to rapid weight gain).
That said ---Do the best you can, try to get her hydrated and eating chick starter - you may need to limit her food intake if you can. Layer feed has too much calcium and not enough protein.
The lethargy is concerning, so keep watch on that. Even at 3wks a meat bird should be moving about some - not as bouncy/hyper as layers. Having a puffed up appearance and unwell look like that could be a sign of Coccidiosis or your chick may something else going on since it has a cross beak (genetic/developmental disorder).
I wish I had better answers for you. Keep us posted.
