Nobody really knows, but I'm gonna state some points here...
> every time a new Volcano appears, a new baby [entei] is born.
So if you analyze that sentence more carefully, you can indicate that there is a whole race of Entei's. it would not a bad guess to say that there is a whole race of Suicune's and Raikou's.
Seeing that's the case, reviving something usually done so the person/thing could come back is it's previous phyical appearance.
Now that we know that, it could be safe to assume that when the 3 dogs died in the fire, they died having the same physical aspects as when the were revived.
No one really knows, but if you gather up your facts and make some guesses in between, you may get the answer.