Again we are talking breeding tinies. In case someone reads who is on the fence, here is MHO. Those who have their minds made up one way or the other can skip over it.
Asking a 3 pound yorkie to pass a possible 6-8 oz pup is like a small framed 100 pound woman having a 15 to 20 pound child. I hate to use humans as examples for dogs but it seems the easiest way to explain.
Yorkies are not uniform in size and unless you know every dog ever in the line for at least 5 generations and their sizes you can't be sure what will be produced. Single pup litters can have a very large pup, and a large litter can lead to fatigue and uterine inertia. A dog with small reserves and a uterus overly stretched could really be at risk.
They also, as Jerry said can and do rupture. This will cost both Dam and pups their lives. This doesn't even speak to the possible lack of pelvic bone space.
Any vet who knows Yorkies are still a relatively young breed with still such varying sizes would never suggest this. If they do, specialist or not I would go elsewhere.