
If you think she looks well, and you known how she normally looks, then I would take the vets opinion with a pinch of salt.
So many know little about breeding and don't realise how tatty their coats get, which makes them look thinner, or some look at a breed weight chart instead of looking and more importantly feeling the body condition.
Most pet dogs are overweight and people get used to seeing that, even vets.
If she has nutritious food, and as much as she wants, then I would not rush anything. If pups want to start eating they will, if not they will when her milk starts to not be enough.
I am curious why you took her for a vet check at 3 weeks after the litter? I personally avoid the vet at all costs when I have pups, as that is where sick animals are, unless something is wrong.