I also agree you need a vet visit for the first day – leave from collecting baby to the vet straight away.
The breeder is irresponsible – selling you ( or attempting to) a bunny at 3 weeks old – not being honest.
If she doesn’t care enough about her health to keep her till right age I would question the health in her rabbitry.
From simple things like mites to serious ones like cocci or ec
I got my Oakley from a great breeder – but he still got poopy butt at weaning. Straight to vet after getting him and no mites , no sign of anything and he has been healthy right through.
Boston was from a bad breeder , who sent him way too young to a bad petstore. Straight to vet with hm and he needed ec treatment which we just finished and they found out today he had microscopic mites and ear mites which he also has now given to Oakley.
Twice during the ec treatment it meant throwing out all his blankets and unwashable toys – bleaching all that I can save
And now with mites we had to do it all again today for both boys. Gets expensive! I don’t regret for a second getting boston – so I’m not saying don’t get her as I know from the other post you are too attached not too- but def go straight to vet, keep her cage decor simple and easy to replace incase something pops up with her health in a week – 10days of getting her as I suspect you’ll need to at least treat something with the sounds of the breeder – and do report her as you said you will after getting her – other baby bun buyers may not be as clued up on ages and health etc and the poor underage bunnies will suffer