You know you still have time to reconsider this idea of letting your 3lb. female have puppies. I recently had my 4 and a half pound girl spayed after her first heat, I was afraid her and my male might have accidently gotten together even though I kept them apart. I couldn't imagine her having puppies and she is quite a bit bigger than yours
I am also surprised that any vet or breeders would suggest breeding such a small dog and so young. Anyway, I would reconsider if it was me, sorry, JMO.