I'm very sorry to hear about the little guy. It's tragic for something like this to happen.
Having two other dogs, you have to be worried now about their welfare. Short of a definitive diagnosis, you'll have to assume it was parvo.
With you having the pup less than a week, it should be assumed that your pup came to you with this disease. This would place the responsibility squarely on the breeder. You likely are protected under California's puppy lemon law. Here's a link with the text of the law.......
Getting a Dog Tips - California's Puppy Lemon Law
The only way I can see where it might not apply is this part...
" (b) Every breeder of dogs shall comply with this article. As used in this article, "dog breeder," or "breeder" means a person, firm, partnership, corporation, or other association that has sold, transferred, or given away all or part of three or more litters or 20 or more dogs during the preceding 12 months that were bred and reared on the premises of the person, firm, partnership, corporation, or other association."
It seems that those that sell less than 3 litters a year are exempt? Sounds like a loophole easily manipulated by shady breeders. The link says the provisions are current as of 2004. Anyone know of any revisions since then?
IMO, parvo should be treated as a public health concern (though not to humans) as it is so easily spread. This breeder could cause a lot of harm....not only to her dogs, but to others dogs, if this remains unchecked. You should not keep quiet about this. Get your documentation from the vet and follow through with this.
Again, very sorry for your loss. I urge you to see your vet immediately about the risks to your other dogs.