Ohhh I am sooo sorry, please give her a big hug and kiss from us all. And I join all the other posters, It is a crime to breed a dog like that, hearing what all she has gone though, not only must they have started to breed her very early, probably also back to back. In my mind a vet like that should be reported, are they not supposed to look to the good of the animal??? And there are pretty strict ethical codes when it comes to breeding, and if a vet is ignoring them this bad, well he need not be a vet. I also am of the very strong oppinion that no earlier than 2 years of age, but with large breeds the window is so small, so if you breed only every other year you would only be able to have 2 litters, one when little over 2 and the second when little over 4, when 6 I would say they are too old to safely birth. But 3 litters all together is a good pointer I think.
I hope everything turn out okay for you and we will be thinking of you both
Marit