Lots of questions regarding what has become of the third egg. Right now it is still in among the chicks. It is hard to see because ot blends in with their bodies, but every so often they separate enough so that you can see it. Most likely f it doesn't hatch, it will eventually be pushed off to the side.
It is not uncommon for an egg to not be viable. When we enter nests to ban birds we sometimes find and unhatched egg. One thing to bear in mind however is that the third egg was laid nearly five days after the second egg...so if you are counting, five days would actually be today. I don't have a whole lot of hope, but I would not completely write it off either.
If the chick does hatch, it will have a very difficult road ahead. His or her siblings will be a lot older and a lot bigger and competition will be a significant factor.
It is going to be hot today so you may see less brooding. You will also probably see the female spending some time sheltering the nestlings from the sun.