I was thinking about this and I am wondering something your vet might be able to answer or stump her to.....
The 1 egg you said has 2 yolks, I could see this egg hatching 2 healthy chicks maybe with some help but they at leased have a chance.
Now for the other 2 eggs you said they shared a yolk.... What happens in the final days when they obsorb the yolk? Will they both get some of it? Will 1 get all of it and 1 get none of it?
I have a theory and its just a theory as I am NOT an expert at hatching but..... I have heard it pretty common with twin eggs that a single chick may survive but the other does not. I am thinking maybe that the reason is they shared a yolk and 1 chick got all the yolk thus had the energy to move and hatch, while the other chick basicly starved before hatching due to lack of nutrition since the other chick had all the yolk...... Just curious what your vet might say about my theory. I don't have any vets around my area that will care for chickens I have found yet to ask this to.
Also I have 3 barred rocks all sisters, 1 of them at 8mths old suddenly went on a double yolk rampage and layed double yolkers for like 2 weeks strait then stopped and hasn't since and the other 2 have never layed double nor has any of my other birds...... I would love to get a double yolker or 2 to try to hatch them but when I am ready for them I get none! Its always the way....... LOL You get em when you don't want em and don't get them when do!