dog miscarriages

consult a good vet.

keep an eye on their diet during pregnancy

Either the female dogs were too young to have a successful pregnancy or your female dogs are all sisters and might have some DNA charateristics that make them unsutable for breeding. Its unlikely to have 3 unrelated females all with this fault but it is very likely if one has that trait, then all the sisters to that female will have it also. If they miscarried twice, do not breed them again as it could be fatal for the female. If a woman miscarried twice, I doubt she would continue without doctors advice. So ask your vet, he/she will definately know the problem and have good advice.

In addition to talking to a vet, I would also try to contact other dog breeders. The best advice comes from people who are out there doing the same thing that you would like to do.